platform/cacheline.c: mark compilation-unit private variables static

Fixes compiler diagnostic:

cacheline.c:31:8: warning: no previous extern declaration for
    non-static variable 'cacheline_size'
    [-Wmissing-variable-declarations]

src/libmowgli/platform/cacheline.c

@@ -28,7 +28,7 @@
 # include <sys/sysctl.h>
 #endif
 
-size_t cacheline_size;
+static size_t cacheline_size = 0;
 
 void
 mowgli_cacheline_bootstrap(void)