sandbox: fix the return type of os_free() function

The function os_free() returns nothing.
Its return type should be "void" rather than "void *".

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>

arch/sandbox/cpu/os.c

@@ -143,7 +143,7 @@ void *os_malloc(size_t length)
 	return hdr + 1;
 }
 
-void *os_free(void *ptr)
+void os_free(void *ptr)
 {
 	struct os_mem_hdr *hdr = ptr;
 

include/os.h

@@ -113,7 +113,7 @@ void *os_malloc(size_t length);
  *
  * \param ptr		Pointer to memory block to free
  */
-void *os_free(void *ptr);
+void os_free(void *ptr);
 
 /**
  * Reallocate previously-allocated memory to increase/decrease space