libc/math : fix warning
libc/math : fix warning

Fixes below warning,

TizenRT/os/include/tinyara/compiler.h:79:20: warning: '__x' may be used
uninitialized in this function [-Wmaybe-uninitialized]
 #define UNUSED(a) ((void)(a))
                    ^
In file included from math/lib_exp2f.c:74:0:
math/libm.h:97:19: note: '__x' was declared here
    volatile float __x;                   \
                   ^
math/lib_exp2f.c:166:5: note: in expansion of macro 'FORCE_EVAL'
     FORCE_EVAL(-0x1p-149f / x);
     ^~~~~~~~~~

Signed-off-by: Manohara HK <02db916c8fbcb3f412fbcb70556ea7b9f661857a@samsung.com>

davidfather/TizenRT,HONGCHAEHEE/TizenRT,pillip8282/TizenRT,jeongarmy/TizenRT,pillip8282/TizenRT,HONGCHAEHEE/TizenRT,Samsung/TizenRT,junmin-kim/TizenRT,sunghan-chang/TizenRT,sunghan-chang/TizenRT,davidfather/TizenRT,junmin-kim/TizenRT,JeonginKim/TizenRT,an4967/TizenRT,pillip8282/TizenRT,jeongchanKim/TizenRT,jeongarmy/TizenRT,chanijjani/TizenRT,Samsung/TizenRT,sunghan-chang/TizenRT,sunghan-chang/TizenRT,sunghan-chang/TizenRT,junmin-kim/TizenRT,JeonginKim/TizenRT,jsdosa/TizenRT,jeongchanKim/TizenRT,Samsung/TizenRT,jsdosa/TizenRT,junmin-kim/TizenRT,jsdosa/TizenRT,an4967/TizenRT,Samsung/TizenRT,HONGCHAEHEE/TizenRT,davidfather/TizenRT,chanijjani/TizenRT,jeongarmy/TizenRT,jeongarmy/TizenRT,chanijjani/TizenRT,HONGCHAEHEE/TizenRT,pillip8282/TizenRT,davidfather/TizenRT,jsdosa/TizenRT,jeongarmy/TizenRT,jeongchanKim/TizenRT,pillip8282/TizenRT,an4967/TizenRT,jsdosa/TizenRT,an4967/TizenRT,HONGCHAEHEE/TizenRT,chanijjani/TizenRT,an4967/TizenRT,Samsung/TizenRT,davidfather/TizenRT,davidfather/TizenRT,Samsung/TizenRT,HONGCHAEHEE/TizenRT,Samsung/TizenRT,jeongchanKim/TizenRT,junmin-kim/TizenRT,jsdosa/TizenRT,pillip8282/TizenRT,JeonginKim/TizenRT,jsdosa/TizenRT,chanijjani/TizenRT,jeongarmy/TizenRT,davidfather/TizenRT,jeongchanKim/TizenRT,an4967/TizenRT,JeonginKim/TizenRT,pillip8282/TizenRT,jeongarmy/TizenRT,sunghan-chang/TizenRT,junmin-kim/TizenRT,jeongchanKim/TizenRT,junmin-kim/TizenRT,sunghan-chang/TizenRT,chanijjani/TizenRT,chanijjani/TizenRT,JeonginKim/TizenRT,JeonginKim/TizenRT,jeongchanKim/TizenRT,an4967/TizenRT