[cflags] Suppress -Wextra-semi, -Wstrict-prototypes

The code doesn't build with these warnings.
Some downstream projects enforce these warnings as errors.

Change-Id: Ic5d4d9e4f2c53cf18dd44604cc4b5d225bf2dc90
diff --git a/BUILD.gn b/BUILD.gn
index 7a100bb..679b60c 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -119,7 +119,10 @@
     "PAGE_SIZE=4096",
   ]
 
+  cflags = [ "-Wextra-semi" ]
+
   cflags_c = [
+    "-Wstrict-prototypes",
     "-Wno-missing-field-initializers",
     "-Wno-newline-eof",
     "-Wno-unused-function",