commit-slab-decl.h: update include guard

When a9f1f1f9f8 ("commit-slab.h: code split", 2018-05-19) split
commit-slab.h into commit-slab-decl.h and commit-slab-impl.h header
files, commit-slab-decl.h were left to use "COMMIT_SLAB_HDR_H",
while commit-slab-impl.h gained its own macro, "COMMIT_SLAB_IMPL_H".

As these two files use different include guards, there is nothing
broken, but let's update commit-slab-decl.h to match the convention
to name the include guard after the filename.

Signed-off-by: Abhishek Kumar <abhishekkumar8222@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

commit-slab-decl.h

@@ -1,5 +1,5 @@
-#ifndef COMMIT_SLAB_HDR_H
-#define COMMIT_SLAB_HDR_H
+#ifndef COMMIT_SLAB_DECL_H
+#define COMMIT_SLAB_DECL_H
 
 /* allocate ~512kB at once, allowing for malloc overhead */
 #ifndef COMMIT_SLAB_SIZE
@@ -40,4 +40,4 @@ elemtype *slabname## _peek(struct slabname *s, const struct commit *c)
 	declare_commit_slab(slabname, elemtype); \
 	declare_commit_slab_prototypes(slabname, elemtype)
 
-#endif /* COMMIT_SLAB_HDR_H */
+#endif /* COMMIT_SLAB_DECL_H */