[debug.h] Introduce a static assert using the array with negative size trick

It is inspired by the COMPILE_ASSERT from WebKit and is using the well
known trick to get compile time assertions.

include/openbsc/debug.h

@@ -19,6 +19,8 @@
 #define DEBUGP(xss, fmt, args...) 
 #endif
 
+#define static_assert(exp, name) typedef int dummy##name [(exp) ? 1 : -1];
+
 void debugp(unsigned int subsys, char *file, int line, const char *format, ...);
 void debug_parse_category_mask(const char* mask);
 void debug_use_color(int use_color);