avoid overlapping "Dummy" symbols

reported by James Jackson in message:
  [Ngspice-users] Problem building git head --with-ngspice Mac OS X

src/maths/misc/isinf.c

@@ -28,6 +28,6 @@ isinf (const double x)
 
 #endif /* HAVE_FINITE */
 #else /* HAVE_ISINF */
-int Dummy_Symbol_5;
+int Dummy_Symbol_7;
 #endif /* HAVE_ISINF */
 #endif /* HAVE_DECL_ISINF */

src/misc/dup2.c

@@ -16,5 +16,5 @@ dup2(int oldd, int newd)
 	return 0;
 }
 #else
-int Dummy_Symbol_2;
+int Dummy_Symbol_6;
 #endif