Replace use of math.h with openlibm.h

src/s_cpow.c

@@ -46,7 +46,7 @@
 
 #include <complex.h>
 #include <float.h>
-#include <math.h>
+#include <openlibm.h>
 
 double complex
 cpow(double complex a, double complex z)

src/s_cpowf.c

@@ -45,7 +45,7 @@
  */
 
 #include <complex.h>
-#include <math.h>
+#include <openlibm.h>
 
 float complex
 cpowf(float complex a, float complex z)

src/s_cpowl.c

@@ -46,7 +46,7 @@
  */
 
 #include <complex.h>
-#include <math.h>
+#include <openlibm.h>
 
 long double complex
 cpowl(long double complex a, long double complex z)