patch 8.1.0592: the libvterm tests are not run as part of Vim tests

Problem:    The libvterm tests are not run as part of Vim tests.
Solution:   Add testing libvterm.
diff --git a/src/libvterm/Makefile b/src/libvterm/Makefile
index 05f68dd..6eacab7 100644
--- a/src/libvterm/Makefile
+++ b/src/libvterm/Makefile
@@ -8,8 +8,7 @@
   LIBTOOL +=--quiet
 endif
 
-# override CFLAGS +=-Wall -Iinclude -std=c99 -DINLINE="static inline" -DUSE_INLINE
-override CFLAGS +=-Wall -Iinclude -std=c90 -Wpedantic -DINLINE=""
+override CFLAGS +=-Wall -Iinclude -std=c99 -Wpedantic -DINLINE=""
 
 ifeq ($(shell uname),SunOS)
   override CFLAGS +=-D__EXTENSIONS__ -D_XPG6 -D__XOPEN_OR_POSIX