build: add support for new build layout for libdispatch

Stage the changes needed in the include path for libdispatch with the
new CMake support.

Author: compnerd

CMakeLists.txt

@@ -98,6 +98,7 @@ add_swift_library(XCTest
 
                     -I${XCTEST_PATH_TO_LIBDISPATCH_SOURCE}
                     -I${XCTEST_PATH_TO_LIBDISPATCH_BUILD}/src/swift
+                    -I${XCTEST_PATH_TO_LIBDISPATCH_BUILD}/src/swift/swift
                     -Xcc -fblocks
 
                     -I${XCTEST_PATH_TO_FOUNDATION_BUILD}/swift