patch 8.1.1891: functions used in one file are global

Problem:    Functions used in one file are global.
Solution:   Add "static". (Yegappan Lakshmanan, closes #4840)
diff --git a/src/gui.c b/src/gui.c
index 1c56745..bb35f51 100644
--- a/src/gui.c
+++ b/src/gui.c
@@ -17,10 +17,13 @@
 static void set_guifontwide(char_u *font_name);
 #endif
 static void gui_check_pos(void);
+static void gui_reset_scroll_region(void);
 static void gui_outstr(char_u *, int);
 static int gui_screenchar(int off, int flags, guicolor_T fg, guicolor_T bg, int back);
+static int gui_outstr_nowrap(char_u *s, int len, int flags, guicolor_T fg, guicolor_T bg, int back);
 static void gui_delete_lines(int row, int count);
 static void gui_insert_lines(int row, int count);
+static int gui_xy2colrow(int x, int y, int *colp);
 #if defined(FEAT_GUI_TABLINE) || defined(PROTO)
 static int gui_has_tabline(void);
 #endif
@@ -1050,7 +1053,7 @@
     return OK;
 }
 
-    void
+    static void
 gui_set_cursor(int row, int col)
 {
     gui.row = row;
@@ -1713,7 +1716,7 @@
 /*
  * Make scroll region cover whole screen.
  */
-    void
+    static void
 gui_reset_scroll_region(void)
 {
     gui.scroll_region_top = 0;
@@ -1722,7 +1725,7 @@
     gui.scroll_region_right = gui.num_cols - 1;
 }
 
-    void
+    static void
 gui_start_highlight(int mask)
 {
     if (mask > HL_ALL)		    /* highlight code */
@@ -2227,7 +2230,7 @@
  * Returns OK, unless "back" is non-zero and using the bold trick, then return
  * FAIL (the caller should start drawing "back" chars back).
  */
-    int
+    static int
 gui_outstr_nowrap(
     char_u	*s,
     int		len,
@@ -3380,7 +3383,7 @@
  * Corrects for multi-byte character.
  * returns column in "*colp" and row as return value;
  */
-    int
+    static int
 gui_xy2colrow(int x, int y, int *colp)
 {
     int		col = check_col(X_2_COL(x));