Fix missing #includes in system/core.

Change-Id: I4736e8c4f7bb8bb03640d26d58243efaa67dc75c
diff --git a/libmemtrack/memtrack.c b/libmemtrack/memtrack.c
index 9a656df..5d68083 100644
--- a/libmemtrack/memtrack.c
+++ b/libmemtrack/memtrack.c
@@ -20,6 +20,7 @@
 
 #include <log/log.h>
 
+#include <errno.h>
 #include <hardware/memtrack.h>
 
 #define ARRAY_SIZE(x) (sizeof(x)/sizeof(x[0]))