Make GDB -Wpointer-sign clean on MinGW too.

Make GDB -Wpointer-sign clean on MinGW too.

This is the remaining issue impeding GDB to build with "-Wpointer-sign
-Werror" with Fedora 17's i686-w64-mingw32 cross toolchain.

../../src/gdb/ser-tcp.c: In function 'net_read_prim':
../../src/gdb/ser-tcp.c:341:3: error: pointer targets in passing argument 2 of 'recv' differ in signedness [-Werror=pointer-sign]
In file included from ../../src/gdb/serial.h:23:0,
                 from ../../src/gdb/ser-tcp.c:21:
/usr/i686-w64-mingw32/sys-root/mingw/include/winsock2.h:983:34: note: expected 'char *' but argument is of type 'unsigned char *'

gdb/
2013-04-19  Pedro Alves  <palves@redhat.com>

	* ser-tcp.c (net_read_prim): Cast second argument to recv to
	'void *'.