test-hashmap.c: drop unnecessary #includes

test-hashmap.c: drop unnecessary #includes

Per Documentation/CodingGuidelines most C files in git start with
a #include of git-compat-util.h or another header file that includes
it, such as cache.h or builtin.h.  This file doesn't need anything
beyond "git-compat-util.h", so use that.

Remove a #include of the system header <stdio.h> since it is already
included by "git-compat-util.h".

Signed-off-by: Jonathan Nieder <redacted>
Signed-off-by: Junio C Hamano <redacted>