math-emu/soft-fp.h: (_FP_ROUND_ZERO) cast 0 to void to fix warning

_FP_ROUND_ZERO is defined as 0 and used as a statemente in macro
_FP_ROUND. This generates "error: statement with no effect
[-Werror=unused-value]" from gcc. Defining _FP_ROUND_ZERO as (void)0 to
fix it.

This modification is quoted from glibc 'commit <In libc/:>
(8ed1e7d5894000c155acbd06f)'
Signed-off-by: Vincent Chen <vincentc@andestech.com>
Acked-by: Greentime Hu <greentime@andestech.com>
Signed-off-by: Greentime Hu <greentime@andestech.com>

include/math-emu/soft-fp.h

@@ -138,7 +138,7 @@ do {							\
       _FP_FRAC_ADDI_##wc(X, _FP_WORK_ROUND);		\
 } while (0)
 
-#define _FP_ROUND_ZERO(wc, X)		0
+#define _FP_ROUND_ZERO(wc, X)		(void)0
 
 #define _FP_ROUND_PINF(wc, X)				\
 do {							\