Make 800 bytes space on IO. From @tridge.

src/modules/px4iofirmware/px4io.h

@@ -48,6 +48,12 @@
 
 #include <systemlib/pwm_limit/pwm_limit.h>
 
+/*
+ hotfix: we are critically short of memory in px4io and this is the
+ easiest way to reclaim about 800 bytes.
+ */
+#define perf_alloc(a,b) NULL
+
 /*
  * Constants and limits.
  */