CRT: Silence -Wunused-but-set-variable warnings in libm.h

Source/Windows/Common/CRT/Musl/libm.h

@@ -153,6 +153,7 @@ static inline long double fp_barrierl(long double x) {
 static inline void fp_force_evalf(float x) {
   volatile float y;
   y = x;
+  (void)y;
 }
 #endif
 
@@ -161,6 +162,7 @@ static inline void fp_force_evalf(float x) {
 static inline void fp_force_eval(double x) {
   volatile double y;
   y = x;
+  (void)y;
 }
 #endif
 
@@ -169,6 +171,7 @@ static inline void fp_force_eval(double x) {
 static inline void fp_force_evall(long double x) {
   volatile long double y;
   y = x;
+  (void)y;
 }
 #endif