Delete useless 'extern' qualifiers for functions

'extern' qualifier is useless for function declarations. Delete
them.

Signed-off-by: Blue Swirl <blauwirbel@gmail.com>
diff --git a/hw/file-op-9p.h b/hw/file-op-9p.h
index c7731c2..126e60e 100644
--- a/hw/file-op-9p.h
+++ b/hw/file-op-9p.h
@@ -57,7 +57,7 @@
     struct xattr_operations **xops;
 } FsContext;
 
-extern void cred_init(FsCred *);
+void cred_init(FsCred *);
 
 typedef struct FileOperations
 {