patch 8.1.2331: the option.c file is still very big

Problem:    The option.c file is still very big.
Solution:   Move a few functions to where they fit better. (Yegappan
            Lakshmanan, closes #4895)
diff --git a/src/proto/term.pro b/src/proto/term.pro
index b29eace..5af1996 100644
--- a/src/proto/term.pro
+++ b/src/proto/term.pro
@@ -25,6 +25,7 @@
 void term_set_winsize(int height, int width);
 void term_fg_color(int n);
 void term_bg_color(int n);
+char_u *term_bg_default(void);
 void term_fg_rgb_color(guicolor_T rgb);
 void term_bg_rgb_color(guicolor_T rgb);
 void term_settitle(char_u *title);