Rename textual includes as *.inc.

This makes it easier to distinguish source files that can stand on their own vs files meant to be #included into other source files.

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2184143002

Review-Url: https://codereview.chromium.org/2184143002
diff --git a/tools/sk_tool_utils_font.cpp b/tools/sk_tool_utils_font.cpp
index 8f701c2..8a7ab46 100644
--- a/tools/sk_tool_utils_font.cpp
+++ b/tools/sk_tool_utils_font.cpp
@@ -15,10 +15,10 @@
 
 namespace sk_tool_utils {
 
-#include "test_font_monospace.cpp"
-#include "test_font_sans_serif.cpp"
-#include "test_font_serif.cpp"
-#include "test_font_index.cpp"
+#include "test_font_monospace.inc"
+#include "test_font_sans_serif.inc"
+#include "test_font_serif.inc"
+#include "test_font_index.inc"
 
 void release_portable_typefaces() {
     for (int index = 0; index < gTestFontsCount; ++index) {