mingw: avoid linking to the C library's isalpha()

mingw: avoid linking to the C library's isalpha()

The implementation of mingw_skip_dos_drive_prefix() calls isalpha() via
has_dos_drive_prefix(). Since the definition occurs long before isalpha()
is defined in git-compat-util.h, my build environment reports:

    CC alloc.o
In file included from git-compat-util.h:186,
                 from cache.h:4,
                 from alloc.c:12:
compat/mingw.h: In function 'mingw_skip_dos_drive_prefix':
compat/mingw.h:365: warning: implicit declaration of function 'isalpha'

Dscho does not see a similar warning in his build and suspects that
ctype.h is included somehow behind the scenes. This implies that his build
links to the C library's isalpha() and does not use git's isalpha().

To fix both the warning in my build and the inconsistency in Dscho's
build, move the function definition to mingw.c. Then it picks up git's
isalpha() because git-compat-util.h is included at the top of the file.

Signed-off-by: Johannes Sixt <j6t@kdbg.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>