Make rpmGenSignature() static

- Having it in signature.h is just wrong as it lives in a different
  library now, and nothing outside rpmgensig.c needs it anymore.
  Should've really been in commit 23a9d64ea4.

lib/rpmgensig.c

@@ -324,7 +324,7 @@ exit:
     return ret;
 }
 
-int rpmGenSignature(Header sigh, const char * file, rpmSigTag sigTag,
+static int rpmGenSignature(Header sigh, const char * file, rpmSigTag sigTag,
 		const char * passPhrase)
 {
     uint8_t * pkt = NULL;

lib/signature.h

@@ -66,17 +66,6 @@ int rpmWriteSignature(FD_t fd, Header h);
  */
 int rpmGenDigest(Header sigh, const char * file, rpmSigTag sigTag);
 
-/** \ingroup signature
- * Generate signature(s) from a header+payload file, save in signature header.
- * @param sigh		signature header
- * @param file		header+payload file name
- * @param sigTag	type of signature(s) to add
- * @param passPhrase	private key pass phrase
- * @return		0 on success, -1 on failure
- */
-int rpmGenSignature(Header sigh, const char * file,
-		    rpmSigTag sigTag, const char * passPhrase);
-
 /** \ingroup signature
  * Verify a signature from a package.
  *