compat/mingw.c: Fix some sparse warnings

compat/mingw.c: Fix some sparse warnings

Sparse issues the following warnings:

        SP compat/mingw.c
    compat/mingw.c:795:3: warning: symbol 'pinfo_t' was not declared. \
        Should it be static?
    compat/mingw.c:796:16: warning: symbol 'pinfo' was not declared. \
        Should it be static?
    compat/mingw.c:797:18: warning: symbol 'pinfo_cs' was not declared. \
        Should it be static?
    compat/mingw.c:1207:23: warning: Using plain integer as NULL pointer

In 'pinfo_t' variable, defined on line 795, seems to have been a
mistake (a missing typedef keyword?), so we simply remove it.

The 'pinfo' variable does not require more than file scope, so we
simply add the static modifier to the declaration.

The 'pinfo_cs' variable, in contrast, requires initialisation in the
mingw replacement main() function, so we add an extern declaration to
the compat/mingw.h header file.

The remaining warning is suppressed by replacing the rhs of the
pointer assignment with the NULL pointer literal.

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