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>