misc/cgo/testsovar: needs the `extern` keyword to export variables on non-windows platforms

Change-Id: I87fa25214fbf24469148a63f4e1e61e261105c16
Reviewed-on: https://go-review.googlesource.com/12470
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

misc/cgo/testsovar/cgoso_c.h

@@ -11,7 +11,7 @@
 #    define VAR __declspec(dllimport)
 #endif
 #else
-#    define VAR
+#    define VAR extern
 #endif
 
 VAR const char *exported_var;