Add missing string protos.

 * src/guess.c:150:12: warning: implicit declaration of function ‘strncmp’ [-Wimplicit-function-declaration]
 * src/guess.c:218:22: warning: implicit declaration of function ‘strcmp’ [-Wimplicit-function-declaration]
 * src/guesslocale.c:42:9: warning: implicit declaration of function ‘toupper’ [-Wimplicit-function-declaration]

--- a/src/guess.c
+++ b/src/guess.c
@@ -21,6 +21,7 @@
  */
 
 #include <stdio.h>
+#include <string.h> /* strcmp */
 
 #include <import.h>
 #include <decode.h>
--- a/src/guesslocale.c
+++ b/src/guesslocale.c
@@ -29,6 +29,7 @@
 #include <langinfo.h>
 #endif
 
+#include <ctype.h> /* toupper */
 #include <import.h>
 #include <ctable.h>
 #include <begin.h>