[osal]: match system implementation with compiler marco

It would make the build system more simple, and the cross
toolchains would do the rest to you.

Toolchains like buildroot default not enable the __gnu_linux__
marco in the pre-processor will surfure from the systsm unknown
problem. You should enable the marco in building toolchains
or have a look on those scripts in build directory.

Change-Id: I8f6af3391aa3d94d602ec8d84ba5e43839859169
Signed-off-by: Randy Li <randy.li@rock-chips.com>

osal/linux/os_log.c

@@ -14,6 +14,7 @@
  * limitations under the License.
  */
 
+#if defined(__gnu_linux__)
 #include <stdio.h>
 #include <stdarg.h>
 #include "os_log.h"
@@ -34,3 +35,4 @@ void os_err(const char* tag, const char* msg, va_list list)
     vfprintf(stderr, line, list);
 }
 
+#endif