patch 8.1.2005: the regexp.c file is too big

Problem:    The regexp.c file is too big.
Solution:   Move the backtracking engine to a separate file. (Yegappan
            Lakshmanan, closes #4905)
diff --git a/Filelist b/Filelist
index d709311..1ed1ecd 100644
--- a/Filelist
+++ b/Filelist
@@ -92,6 +92,7 @@
 		src/profiler.c \
 		src/quickfix.c \
 		src/regexp.c \
+		src/regexp_bt.c \
 		src/regexp_nfa.c \
 		src/regexp.h \
 		src/scriptfile.c \