Fortran: Remove unused declaration

This function was removed years ago, remove it's prototype.

gcc/fortran/ChangeLog:

	* gfortran.h (gfc_check_include): Remove declaration.
diff --git a/gcc/fortran/gfortran.h b/gcc/fortran/gfortran.h
index 6bd8800..b541a07 100644
--- a/gcc/fortran/gfortran.h
+++ b/gcc/fortran/gfortran.h
@@ -3207,7 +3207,6 @@
 int gfc_at_bol (void);
 int gfc_at_eol (void);
 void gfc_advance_line (void);
-int gfc_check_include (void);
 int gfc_define_undef_line (void);
 
 int gfc_wide_is_printable (gfc_char_t);