exclude cgms.cpp from DM on Google3 build

Having a mix of C and C++ sources in DM makes setting build flags
tricky.  This removes the C sources.  I don't think it's important
to run these C-based GMs in Google3... I don't even really think
it's terribly important to run DM there at all.

Change-Id: I2c2653ee15981f9a63965fb8b7e483a5c5e94830
Reviewed-on: https://skia-review.googlesource.com/131441
Reviewed-by: Mike Klein <mtklein@chromium.org>
Commit-Queue: Mike Klein <mtklein@chromium.org>

public.bzl

@@ -433,7 +433,6 @@ DM_SRCS_ALL = struct(
         "dm/*.h",
         "experimental/svg/model/*.cpp",
         "experimental/svg/model/*.h",
-        "gm/*.c",
         "gm/*.cpp",
         "gm/*.h",
         "src/xml/*.cpp",
@@ -489,6 +488,7 @@ DM_SRCS_ALL = struct(
         "tools/trace/*.h",
     ],
     exclude = [
+        "gm/cgms.cpp",
         "tests/FontMgrAndroidParserTest.cpp",  # Android-only.
         "tests/skia_test.cpp",  # Old main.
         "tools/gpu/atlastext/*",