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
  */