Use correct c++ standard library in Xcode project.

This makes it build again for Xcode 10.

Change-Id: Ia28b8c0b131ac2862a8c17e223d1d7b1f0f59134
Reviewed-on: https://chromium-review.googlesource.com/c/breakpad/breakpad/+/1574177
Reviewed-by: Mark Mentovai <mark@chromium.org>
diff --git a/src/client/ios/Breakpad.xcodeproj/project.pbxproj b/src/client/ios/Breakpad.xcodeproj/project.pbxproj
index e047b51..132670e 100644
--- a/src/client/ios/Breakpad.xcodeproj/project.pbxproj
+++ b/src/client/ios/Breakpad.xcodeproj/project.pbxproj
@@ -448,6 +448,7 @@
 			isa = XCBuildConfiguration;
 			buildSettings = {
 				ALWAYS_SEARCH_USER_PATHS = NO;
+				CLANG_CXX_LIBRARY = "libc++";
 				COPY_PHASE_STRIP = NO;
 				DSTROOT = /tmp/Breakpad.dst;
 				FRAMEWORK_SEARCH_PATHS = (
@@ -479,6 +480,7 @@
 			isa = XCBuildConfiguration;
 			buildSettings = {
 				ALWAYS_SEARCH_USER_PATHS = NO;
+				CLANG_CXX_LIBRARY = "libc++";
 				DSTROOT = /tmp/Breakpad.dst;
 				FRAMEWORK_SEARCH_PATHS = (
 					"$(inherited)",