libc: Make no-debug assert() conform to C requirements

The C standard requires assert() to be a void result, so you
could write something like:

  return assert(x), x;

From the C11 standard (7.2 Diagnostic <assert.h>):
> If NDEBUG is defined as a macro name at the point in the source file
> where <assert.h> is included, the assert macro is defined simply as
>    #define assert(ignore) ((void)0)

Signed-off-by: Thiago Macieira <thiago.macieira@intel.com>

lib/libc/minimal/include/assert.h

@@ -20,7 +20,7 @@ extern "C" {
 #endif
 #else
 #ifndef assert
-#define assert(test)
+#define assert(test) ((void)0)
 #endif
 #endif