Avoid redefining MIN

In file included from build/pclient/main.c:13:
./prio/util.h:17:9: error: 'MIN' macro redefined [-Werror,-Wmacro-redefined]
 #define MIN(a, b) ((a) < (b) ? (a) : (b))
         ^
/usr/include/sys/param.h:308:9: note: previous definition is here
 #define MIN(a,b) (((a)<(b))?(a):(b))
         ^

prio/util.h

@@ -13,7 +13,9 @@
 #include <mprio.h>
 
 // Minimum of two values
+#ifndef MIN
 #define MIN(a, b) ((a) < (b) ? (a) : (b))
+#endif
 
 // Check a Prio error code and return failure if the call fails.
 #define P_CHECK(s)                                                             \