Prepare for |is_posix| switch in the Fuchsia build in boringssl/

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

Bug: 812974
Change-Id: Ic0292e13421cecacd54f49a981778cda746c08f0
Reviewed-on: https://chromium-review.googlesource.com/988839Reviewed-by: Adam Langley <agl@chromium.org>
Commit-Queue: Fabrice de Gans-Riberi <fdegans@chromium.org>
Cr-Commit-Position: refs/heads/master@{#548179}

third_party/boringssl/BUILD.gn

@@ -33,7 +33,7 @@ config("internal_config") {
     "//build/config/compiler:no_size_t_to_int_warning",
     "//build/config/sanitizers:cfi_icall_generalize_pointers",
   ]
-  if (is_posix) {
+  if (is_posix || is_fuchsia) {
     cflags_c = [ "-std=c99" ]
     defines += [ "_XOPEN_SOURCE=700" ]
   }