Fix GDB build under msys+mingw gcc 32bit

Fix GDB build under msys+mingw gcc 32bit

I see a build error when building GDB under msys+mingw gcc 32bit:

 g++ -x c++ -std=gnu++11 -g -O2    -I. -I../../../binutils-gdb/gdb/gdbserver -I../../../binutils-gdb/gdb/gdbserver/../common -I../../../binutils-gdb/gdb/gdbserver/../regformats -I../../../binutils-gdb/gdb/gdbserver/.. -I../../../binutils-gdb/gdb/gdbserver/../../include -I../../../binutils-gdb/gdb/gdbserver/../gnulib/import -Ibuild-gnulib-gdbserver/import  -Wall -Wpointer-arith -Wno-unused -Wunused-value -Wunused-function -Wno-switch -Wno-char-subscripts -Wempty-body -Wunused-but-set-parameter -Wunused-but-set-variable -Wno-sign-compare -Wno-narrowing -Wno-error=maybe-uninitialized -Wno-format -Werror -DGDBSERVER -c -o win32-low.o -MT win32-low.o -MMD -MP -MF .deps/win32-low.Tpo ../../../binutils-gdb/gdb/gdbserver/win32-low.c
 ../../../binutils-gdb/gdb/gdbserver/win32-low.c: In function 'BOOL create_process(const char*, char*, DWORD, PROCESS_INFORMATION*)':
 ../../../binutils-gdb/gdb/gdbserver/win32-low.c:566:48: error: 'get_inferior_cwd' was not declared in this scope
    const char *inferior_cwd = get_inferior_cwd ();
						 ^
 make[4]: *** [win32-low.o] Error 1

It can be fixed by simply including the right header file.

gdb/gdbserver/ChangeLog:
2017-10-06  Yuanhui Zhang  <asmwarrior@gmail.com>

	* win32-low.c: Include "common-inferior.h".