Try to catch missing std includes on travis

By defining LWIP_PLATFORM_ASSERT to a function, we can prevent arch.h
from including stdio.h and stdlib.h

Signed-off-by: Simon Goldschmidt <goldsimon@gmx.de>

contrib/examples/example_app/test.c

@@ -760,3 +760,13 @@ int main(void)
 
   return 0;
 }
+
+/* This function is only required to prevent arch.h including stdio.h
+ * (which it does if LWIP_PLATFORM_ASSERT is undefined)
+ */
+void lwip_example_app_platform_assert(const char *msg, int line, const char *file)
+{
+  printf("Assertion \"%s\" failed at line %d in %s\n", msg, line, file);
+  fflush(NULL);
+  abort();
+}