Silence unguarded availability warning from Xcode 8.3

Summary: Silence unguarded availability warning that appeared in Xcode 8.3 when importing `<XCTest/XCTest.h>`. This allows WDA to compile by silencing the following error: ``` While building module 'XCTest' imported from WebDriverAgent/WebDriverAgentLib/Categories/XCUIApplication+FBHelpers.h:10: In file included from <module-includes>:1: In file included from /Applications/Xcode.app/Contents/Developer/Platforms/iPhoneSimulator.platform/Developer/Library/Frameworks/XCTest.framework/Headers/XCTest.h:57: /Applications/Xcode.app/Contents/Developer/Platforms/iPhoneSimulator.platform/Developer/Library/Frameworks/XCTest.framework/Headers/XCUIDevice.h:50:22: error: 'XCUISiriService' is partial: introduced in iOS 10.3 [-Werror,-Wunguarded-availability] property (readonly) XCUISiriService *siriService NS_AVAILABLE_IOS(10_3); ^ While building module 'XCTest' imported from WebDriverAgent/WebDriverAgentLib/Categories/XCUIApplication+FBHelpers.h:10: In file included from <module-includes>:1: In fi Closes facebookarchive#525 Differential Revision: D4818640 Pulled By: marekcirkos fbshipit-source-id: 6734852fc20714a67fd7e918f551415344cd6951

Author: Michael Petrov

Configurations/ProjectSettings.xcconfig

@@ -24,4 +24,4 @@ CLANG_WARN_SUSPICIOUS_IMPLICIT_CONVERSION = YES
 
 GCC_PREPROCESSOR_DEFINITIONS = $(inherited)
 
-WARNING_CFLAGS = $(inherited) -Weverything -Wno-objc-missing-property-synthesis -Wno-unused-macros -Wno-disabled-macro-expansion -Wno-gnu-statement-expression -Wno-language-extension-token -Wno-overriding-method-mismatch -Wno-missing-variable-declarations -Rno-module-build -Wno-auto-import -Wno-objc-interface-ivars -Wno-documentation-unknown-command -Wno-reserved-id-macro -Wno-unused-parameter -Wno-gnu-conditional-omitted-operand -Wno-explicit-ownership-type -Wno-date-time -Wno-cast-align -Wno-cstring-format-directive -Wno-double-promotion
+WARNING_CFLAGS = $(inherited) -Weverything -Wno-objc-missing-property-synthesis -Wno-unused-macros -Wno-disabled-macro-expansion -Wno-gnu-statement-expression -Wno-language-extension-token -Wno-overriding-method-mismatch -Wno-missing-variable-declarations -Rno-module-build -Wno-auto-import -Wno-objc-interface-ivars -Wno-documentation-unknown-command -Wno-reserved-id-macro -Wno-unused-parameter -Wno-gnu-conditional-omitted-operand -Wno-explicit-ownership-type -Wno-date-time -Wno-cast-align -Wno-cstring-format-directive -Wno-double-promotion -Wno-partial-availability