Prepare for |is_posix| switch in the Fuchsia build

|is_posix| will be switched to false for Fuchsia, this is a preliminary change.

Bug: chromium:812974
Change-Id: Iabe2503db3d6d4cebf1445cb4ab0852b664e1dba
Reviewed-on: https://pdfium-review.googlesource.com/29490
Reviewed-by: dsinclair <dsinclair@chromium.org>
Commit-Queue: dsinclair <dsinclair@chromium.org>
diff --git a/BUILD.gn b/BUILD.gn
index 1cb7046..b4e83a4 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -796,7 +796,7 @@
       "third_party:png",
     ]
   }
-  if (is_posix) {
+  if (is_posix || is_fuchsia) {
     # core/fxcodec/fx_libopenjpeg/src/fx_mct.c does an pointer-to-int
     # conversion to check that an address is 16-bit aligned (benign).
     cflags_c = [ "-Wno-pointer-to-int-cast" ]