submodule-config.h: remove unnecessary include

The unnecessary include in the header transitively pulled in some
other headers actually needed by source files, though.  Have those
source files explicitly include the headers they need.

Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

submodule-config.h

@@ -3,7 +3,6 @@
 
 #include "config.h"
 #include "submodule.h"
-#include "strbuf.h"
 #include "tree-walk.h"
 
 /**

t/helper/test-submodule.c

@@ -4,6 +4,7 @@
 #include "remote.h"
 #include "repository.h"
 #include "setup.h"
+#include "strbuf.h"
 #include "submodule-config.h"
 #include "submodule.h"