Enable the use of common blocks

Vim is a legacy codebase that relies on this behavior.

Change-Id: I8eb57c6698c76e1459fa29b931bee96e44c9ee1f
diff --git a/BUILD.gn b/BUILD.gn
index 505014c..b6558f7 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -6,6 +6,7 @@
 
 config("vim_config") {
   cflags = [
+    "-fcommon",
     "-Wno-unused-function",
     "-Wno-shift-op-parentheses",
     "-Wno-dangling-else",