Avoid implicit function declarations in lang.c.  Future compilers will
not support them by default, leading to build failures.

Submitted upstream: <https://gitlab.com/alevt/alevt/-/merge_requests/10>

diff --git a/lang.c b/lang.c
index 847fd52a7d2d1142..9882dc85a7b08929 100644
--- a/lang.c
+++ b/lang.c
@@ -4,6 +4,9 @@
 #include "vt.h"
 #include "lang.h"
 
+static int is_greek(int c);
+static int is_koi(int c);
+
 int latin1 = -1;
 
 static u8 lang_char[256];
@@ -197,14 +200,14 @@ else if ((latin1 == GREEK) && lang==15) { /* Hellas */
 }
 
 /* check for Greek chars - needs locale iso8859-7 set */
-int is_greek(int c)
+static int is_greek(int c)
 {
   if( isalpha(c | 0x80)) return 1;
   return 0;
 }
 
 /* check for russian chars - needs locale KOI8-R set */
-int is_koi(int c)
+static int is_koi(int c)
 {
   if( isalpha(c | 0x80)) return 1;
   if( c=='&' ) return 1;