Teach mgrep to find soong/*.go files.

Test: $ cd $ANDROID_BUILD_TOP
      $ mgrep FORCE_BUILD_LLVM_DEBUG
      ./external/llvm/llvm-host-build.mk:24:ifeq ($(FORCE_BUILD_LLVM_DEBUG),true)
      ./external/llvm/soong/llvm.go:44:       if ctx.AConfig().IsEnvTrue("FORCE_BUILD_LLVM_DEBUG") {
      $ cd external/llvm
      $ mgrep FORCE_BUILD_LLVM_DEBUG
      ./llvm-host-build.mk:24:ifeq ($(FORCE_BUILD_LLVM_DEBUG),true)
      ./soong/llvm.go:44:     if ctx.AConfig().IsEnvTrue("FORCE_BUILD_LLVM_DEBUG") {

Change-Id: Ibe114550a8022d792a0c10c766c964a67721a6ba