Create a GN target to build all variants

This is necessary so we can build all FFmpeg variants as prebuilts.

Bug: BLD-197
Change-Id: I8f02eaaf02f5b71f055272eff7582d6d2557cc53
diff --git a/BUILD.gn b/BUILD.gn
index cdfe3ff..92048d5 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -208,3 +208,10 @@
     deps += [ ":ffmpeg_yasm" ]
   }
 }
+
+group("ffmpeg_variants") {
+  deps = []
+  foreach(variant, select_variant) {
+    deps += [ ":ffmpeg(${toolchain_variant.base}-${variant}-shared)" ]
+  }
+}