Backport fix for newer versions of glibc to resolve major() minor() definition issues

[btashton]

This backports https://github.com/qemu/qemu/commit/4d04351f4c3db3b70dc21f7fdc8155e341f39916 This allows us to build with modern version of glibc.

For example on my Fedora 33:

❯ ./configure --target-list=mipsel-softmmu --python=/usr/bin/python2 --disable-werror
❯ ./mipsel-softmmu/qemu-system-mipsel   -machine pic32mx7-explorer16   -nographic -monitor none   -serial stdio   -kernel  ~/nuttx/wrk/nuttx/nuttx.hex 
Board: Microchip Explorer16
Processor: M4K
RAM size: 128 kbytes
Load file: '/home/bashton/nuttx/wrk/nuttx/nuttx.hex', 250268 bytes
nx_start: Entry

Original patch context

The definition of the major() and minor() macros are moving within glibc to
<sys/sysmacros.h>. Include this header when it is available to avoid the
following sorts of build-stopping messages:

qga/commands-posix.c: In function ‘dev_major_minor’:
qga/commands-posix.c:656:13: error: In the GNU C Library, "major" is defined
 by <sys/sysmacros.h>. For historical compatibility, it is
 currently defined by <sys/types.h> as well, but we plan to
 remove this soon. To use "major", include <sys/sysmacros.h>
 directly. If you did not intend to use a system-defined macro
 "major", you should undefine it after including <sys/types.h>. [-Werror]
         *devmajor = major(st.st_rdev);
             ^~~~~~~~~~~~~~~~~~~~~~~~~~

qga/commands-posix.c:657:13: error: In the GNU C Library, "minor" is defined
 by <sys/sysmacros.h>. For historical compatibility, it is
 currently defined by <sys/types.h> as well, but we plan to
 remove this soon. To use "minor", include <sys/sysmacros.h>
 directly. If you did not intend to use a system-defined macro
 "minor", you should undefine it after including <sys/types.h>. [-Werror]
         *devminor = minor(st.st_rdev);