Clean up a change that I sneaked in accidentically.

I often build the code with -Wmissing-prototypes to ensure that we don't
accidentically pollute the symbol namespace. If we want to provide a
symbol such as isopenlibm(), make sure we also declare it in
<openlibm_math.h>.

include/openlibm_math.h

@@ -194,6 +194,9 @@ extern int signgam;
 #if defined(__cplusplus)
 extern "C" {
 #endif
+/* Symbol present when OpenLibm is used. */
+int isopenlibm(void);
+
 /*
  * ANSI/POSIX
  */

src/common.c

@@ -1,6 +1,6 @@
-#include "math_private.h"
+#include <openlibm_math.h>
 
-DLLEXPORT int isopenlibm(void);
+#include "math_private.h"
 
 DLLEXPORT int isopenlibm(void) {
     return 1;