build: include sys/sysmacros.h for major() and minor()

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);
             ^~~~~~~~~~~~~~~~~~~~~~~~~~

The additional include allows the build to complete on Fedora 26 (Rawhide)
with glibc version 2.24.90.

Signed-off-by: Christopher Covington <cov@codeaurora.org>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>

include/sysemu/os-posix.h

@@ -34,6 +34,10 @@
 #include <netdb.h>
 #include <sys/un.h>
 
+#ifdef CONFIG_SYSMACROS
+#include <sys/sysmacros.h>
+#endif
+
 void os_set_line_buffering(void);
 void os_set_proc_name(const char *s);
 void os_setup_signal_handling(void);