compat/nedmalloc: Fix some sparse warnings

compat/nedmalloc: Fix some sparse warnings

Sparse issues many "Using plain integer as NULL pointer" warnings
while checking nedmalloc.c (at least 98 such warnings before giving
up due to "too many warnings"). In addition, sparse issues some
"non-ANSI function declaration" type warnings for the symbols
'win32_getcurrentthreadid', 'malloc_stats' and 'malloc_footprint'.

In order to suppress the NULL pointer warnings, rather than replace
all uses of '0' as a null pointer representation with NULL, we add
-Wno-non-pointer-null to SPARSE_FLAGS while checking nedmalloc.c.

In order to suppress the "non-ANSI function declaration" warnings,
we simply include the missing 'empty parameter list' prototype (void)
in the function declarations.

Signed-off-by: Ramsay Jones <ramsay@ramsay1.demon.co.uk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>