Fix build with clang

Building with clang and -Werror option imposed by fuchsia BUILD.gn results in error due to -Wpointer-bool-conversion.
Proposed solution to supress this warnings with -Wno-pointer-bool-conversion when building for fuchsia using clang.

Change-Id: Idb4d44c50b4398e49e86cbbcefc19533fa43c285

BUILD.gn

diff --git a/BUILD.gn b/BUILD.gn
index e710064..dd55576 100755
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -126,7 +126,7 @@
 config("ffmpeg_fuchsia_warnings") {
   cflags = []
   if (is_clang) {
-    cflags += [ "-Wno-sign-compare", "-Wno-missing-field-initializers" ]
+    cflags += [ "-Wno-sign-compare", "-Wno-missing-field-initializers", "-Wno-pointer-bool-conversion" ]
   }
 }