compat/nedmalloc: Fix compiler warnings on linux

compat/nedmalloc: Fix compiler warnings on linux

On linux, when the build variable USE_NED_ALLOCATOR is set, gcc
issues the following warnings:

    In file included from compat/nedmalloc/nedmalloc.c:63:
    .../malloc.c.h: In function 'mmap_resize':
    .../malloc.c.h:3762: warning: implicit declaration of function 'mremap'
    .../malloc.c.h: In function 'sys_trim':
    .../malloc.c.h:4195: warning: comparison between pointer and integer

The warnings are caused by the <sys/mman.h> header not enabling the
(conditional) declaration of the mremap() function.  The declaration
can be enabled by defining the _GNU_SOURCE symbol prior to including
certain system header files. In particular, it may not be sufficient
to simply define _GNU_SOURCE just prior to including the <sys/mman.h>
header. (e.g. defining the symbol after including <sys/types.h> will
be completely ineffective.)

In order to suppress the warnings, we define the _GNU_SOURCE symbol
at the start of the malloc.c.h header file.

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