patch 8.0.1182: cannot see or change mzscheme dll name

Problem:    Cannot see or change mzscheme dll name.
Solution:   Add 'mzschemedll' and 'mzschemegcdll'.
diff --git a/src/option.h b/src/option.h
index ecd70dc..88eadae 100644
--- a/src/option.h
+++ b/src/option.h
@@ -662,6 +662,10 @@
 EXTERN int	p_more;		/* 'more' */
 #ifdef FEAT_MZSCHEME
 EXTERN long	p_mzq;		/* 'mzquantum */
+# if defined(DYNAMIC_MZSCHEME)
+EXTERN char_u	*p_mzschemedll;	/* 'mzschemedll' */
+EXTERN char_u	*p_mzschemegcdll; /* 'mzschemegcdll' */
+# endif
 #endif
 #if defined(MSWIN)
 EXTERN int	p_odev;		/* 'opendevice' */