locate: fix -fno-common build

Just a single variable declaration to extern and define elsewhere here,
myctype.

-fno-common will become a default in GCC10/LLVM11.

MFC after:	3 days

usr.bin/locate/locate/locate.c

@@ -116,6 +116,7 @@ int f_limit;            /* limit number of output lines, 0 == infinite */
 u_int counter;          /* counter for matches [-c] */
 char separator='\n';	/* line separator */
 
+u_char myctype[UCHAR_MAX + 1];
 
 void    usage(void);
 void    statistic(FILE *, char *);

usr.bin/locate/locate/locate.h

@@ -65,7 +65,7 @@
    define TOLOWER(ch)	  tolower(ch)
 #else
 
-u_char myctype[UCHAR_MAX + 1];
+extern u_char myctype[UCHAR_MAX + 1];
 #define TOLOWER(ch)	(myctype[ch])
 #endif