kram - turn on precompiled pch for libkram build for Xcode

KramPrefix.pch is only including KramConfig.h for now.   Still need to enable in Cmake for Win build.  I also ran Aras's build analysis tool on the json output of -ftime-trace.  It reported 11s vs 78s of parsing time.  That may be spread across the 6+2 cores on my machine.

Here are M1 (6/2) timings for PCH vs. not from Xcode build timings.
Debug (arm64)  7s vs. 16s
Release (arm64+x64) 33s vs. 44s.

build2/kram.xcodeproj/project.pbxproj

@@ -1908,7 +1908,6 @@
 				CLANG_X86_VECTOR_INSTRUCTIONS = avx;
 				CODE_SIGN_STYLE = Automatic;
 				EXECUTABLE_PREFIX = lib;
-				GCC_PRECOMPILE_PREFIX_HEADER = NO;
 				PRODUCT_NAME = "$(TARGET_NAME)";
 				SKIP_INSTALL = YES;
 				SYSTEM_HEADER_SEARCH_PATHS = "";
@@ -1923,7 +1922,6 @@
 				CLANG_X86_VECTOR_INSTRUCTIONS = avx;
 				CODE_SIGN_STYLE = Automatic;
 				EXECUTABLE_PREFIX = lib;
-				GCC_PRECOMPILE_PREFIX_HEADER = NO;
 				PRODUCT_NAME = "$(TARGET_NAME)";
 				SKIP_INSTALL = YES;
 				SYSTEM_HEADER_SEARCH_PATHS = "";
@@ -1938,7 +1936,6 @@
 				CLANG_X86_VECTOR_INSTRUCTIONS = default;
 				CODE_SIGN_STYLE = Automatic;
 				EXECUTABLE_PREFIX = lib;
-				GCC_PRECOMPILE_PREFIX_HEADER = NO;
 				IPHONEOS_DEPLOYMENT_TARGET = 14.1;
 				PRODUCT_NAME = "$(TARGET_NAME)";
 				SDKROOT = iphoneos;
@@ -1956,7 +1953,6 @@
 				CLANG_X86_VECTOR_INSTRUCTIONS = default;
 				CODE_SIGN_STYLE = Automatic;
 				EXECUTABLE_PREFIX = lib;
-				GCC_PRECOMPILE_PREFIX_HEADER = NO;
 				IPHONEOS_DEPLOYMENT_TARGET = 14.1;
 				PRODUCT_NAME = "$(TARGET_NAME)";
 				SDKROOT = iphoneos;