Pass -f argument to git fetch

The glslang repo has tags that keep getting updated, and git fetch --tags
was erroring out because it didn't want to allow that.

Change-Id: Ia5e78ce38035d71ae58f06e502c6b7c98f5ff443
diff --git a/external/fetch_sources.py b/external/fetch_sources.py
index ac7abe6..f60aefb 100755
--- a/external/fetch_sources.py
+++ b/external/fetch_sources.py
@@ -286,7 +286,7 @@
 				(stdout, stderr) = proc.communicate()
 				if proc.returncode == 0:
 					execute(["git", "tag", "-d",tag])
-			execute(["git", "fetch", "--tags", url, "+refs/heads/*:refs/remotes/origin/*"])
+			execute(["git", "fetch", "-f", "--tags", url, "+refs/heads/*:refs/remotes/origin/*"])
 			execute(["git", "checkout", self.revision])
 		finally:
 			popWorkingDir()