[build] Set -Wno-strict-prototypes.

https://reviews.llvm.org/D122895 changed the semantics of
-Wstrict-prototypes, which causes -Werror build failures in the core
builders. To fix the warning, this series of changes sets
-Wno-strict-prototypes on affected third-party dependencies.

Error:
  third_party/libffi/src/dlmalloc.c:4450:20: error: a function
  declaration without a prototype is deprecated in all versions of C

Bug: 97746
Change-Id: I27e78e33516fd2c99725ea019fb3c97f8f8a5970
diff --git a/BUILD.gn b/BUILD.gn
index ba3f2f6..e92086e 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -59,6 +59,8 @@
     # For asm keyword in src/aarch64/ffi.c
     "-std=gnu11",
   ]
+
+  cflags_c = [ "-Wno-strict-prototypes" ]
 }
 
 source_set("ffi_static") {