configure: fix mingw32 libs_qga typo

It's typical to prepend or append parameters to an argument string so
that other places in ./configure can add parameters without clobbering
the string.  In the mingw32 libs_qga case there is a typo "$lib_qga"
instead of "$libs_qga".

Signed-off-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>
Reviewed-by: Andreas Färber <afaerber@suse.de>

configure

@@ -526,7 +526,7 @@ EOF
   bindir="\${prefix}"
   sysconfdir="\${prefix}"
   confsuffix=""
-  libs_qga="-lws2_32 -lwinmm -lpowrprof $lib_qga"
+  libs_qga="-lws2_32 -lwinmm -lpowrprof $libs_qga"
 fi
 
 werror=""