Remove include <log.h> from defines.h

The reason for this is that log.h includes drv_hrt.h which sucks in
half the project's include files with it, which causes strange build
issues

src/examples/px4_simple_app/px4_simple_app.c

@@ -39,6 +39,7 @@
  */
 
 #include <px4_platform_common/px4_config.h>
+#include <px4_platform_common/log.h>
 #include <px4_platform_common/tasks.h>
 #include <px4_platform_common/posix.h>
 #include <unistd.h>