patch 8.1.0023: gcc 8.1 warns for use of strncpy()

Problem:    gcc 8.1 warns for use of strncpy(). (John Marriott)
Solution:   Use mch_memmove() instead of STRNCPY().
diff --git a/src/memline.c b/src/memline.c
index 7d6cefa..be395fc 100644
--- a/src/memline.c
+++ b/src/memline.c
@@ -344,7 +344,7 @@
     b0p->b0_magic_int = (int)B0_MAGIC_INT;
     b0p->b0_magic_short = (short)B0_MAGIC_SHORT;
     b0p->b0_magic_char = B0_MAGIC_CHAR;
-    STRNCPY(b0p->b0_version, "VIM ", 4);
+    mch_memmove(b0p->b0_version, "VIM ", 4);
     STRNCPY(b0p->b0_version + 4, Version, 6);
     long_to_char((long)mfp->mf_page_size, b0p->b0_page_size);