[ppc] avoid empty mdproc struct

Avoid empty structs, that have undefined behavior in C99 and
make compilers complain about it
(empty struct has size 0 in C, size 1 in C++).

Reviewed by:	jhibbits
Differential Revision:	https://reviews.freebsd.org/D21231

sys/powerpc/include/proc.h

@@ -46,6 +46,11 @@ struct mdthread {
 };
 
 struct mdproc {
+	/* Avoid empty structs, that have undefined behavior in C99 and
+	 * make compilers complain about it
+	 * (empty struct has size 0 in C, size 1 in C++).
+	 */
+	long	md_spare;
 };
 
 #ifdef __powerpc64__