patch 8.1.1034: too many #ifdefs

Problem:    Too many #ifdefs.
Solution:   Merge FEAT_MOUSE_SGR into FEAT_MOUSE_XTERM / FEAT_MOUSE_TTY.
diff --git a/src/evalfunc.c b/src/evalfunc.c
index f5d1da7..f5a0bd8 100644
--- a/src/evalfunc.c
+++ b/src/evalfunc.c
@@ -6414,7 +6414,7 @@
 # ifdef FEAT_MOUSE_PTERM
 	"mouse_pterm",
 # endif
-# ifdef FEAT_MOUSE_SGR
+# ifdef FEAT_MOUSE_XTERM
 	"mouse_sgr",
 # endif
 # ifdef FEAT_SYSMOUSE