fix exp10l.c to include float.h

the previous commit was a no op in exp10l because LDBL_* macros
were implicitly 0 (the preprocessor does not warn about undefined
symbols).

src/math/exp10l.c

@@ -1,4 +1,5 @@
 #define _GNU_SOURCE
+#include <float.h>
 #include <math.h>
 #include "libc.h"