Fix Windows GN build.

The compiler file unconditionally depenended on the GCC version.

TBR=scottmg@chromium.org

Review URL: https://codereview.chromium.org/292703002

git-svn-id: http://src.chromium.org/svn/trunk/src/build@271347 4ff67af0-8c30-449e-8e8b-ad334ec8d88c

config/compiler/BUILD.gn

@@ -2,11 +2,13 @@
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
-import("//build/config/gcc/gcc_version.gni")
 import("//build/config/android/config.gni")
 if (cpu_arch == "arm") {
   import("//build/config/arm.gni")
 }
+if (is_posix) {
+  import("//build/config/gcc/gcc_version.gni")
+}
 
 declare_args() {
   # Normally, Android builds are lightly optimized, even for debug builds, to