compat/fnmatch/fnmatch.c: Fix a sparse error

compat/fnmatch/fnmatch.c: Fix a sparse error

Sparse issues the following error and warnings:

    SP compat/fnmatch/fnmatch.c
.../fnmatch.c:144:17: warning: Using plain integer as NULL pointer
.../fnmatch.c:238:67: warning: Using plain integer as NULL pointer
.../fnmatch.c:303:40: error: too many arguments for function getenv

The error is caused by the extern declaration for the getenv function
not including the function prototype. Without the prototype, sparse
effectively sees the declaration as 'char *getenv(void)'. In order to
suppress the error, we simply include the function prototype.

In order to suppress the warnings, we include the <stddef.h> header
which provides an appropriate definition of the NULL macro, rather
than using the (inappropriate) default definition at fnmatch.c:132.

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