build: fix compilation when configured with --enable-tiny

Commit 0c1bf429 from last week added two calls to digits()
for --constantshow.

src/prototypes.h

@@ -532,9 +532,7 @@ void complete_a_word(void);
 void get_homedir(void);
 const char *tail(const char *path);
 char *concatenate(const char *path, const char *name);
-#ifdef ENABLE_LINENUMBERS
 int digits(ssize_t n);
-#endif
 bool parse_num(const char *str, ssize_t *result);
 bool parse_line_column(const char *str, ssize_t *line, ssize_t *column);
 void recode_NUL_to_LF(char *string, size_t length);

src/utils.c

@@ -76,7 +76,6 @@ char *concatenate(const char *path, const char *name)
 	return joined;
 }
 
-#ifdef ENABLE_LINENUMBERS
 /* Return the number of digits that the given integer n takes up. */
 int digits(ssize_t n)
 {
@@ -106,7 +105,6 @@ int digits(ssize_t n)
 		}
 	}
 }
-#endif
 
 /* Read an integer from the given string.  If it parses okay,
  * store it in *result and return TRUE; otherwise, return FALSE. */