Fix included: invalid syntax when setting LV_CFLAGS

[ThomasFarstrike]

Describe the bug

When I set LV_CFLAGS when running make.py, I got a syntax error.

Expected behavior
No syntax error.

I traced the issue to ext_mod/lvgl/micropython.mk

Here's the diff that fixed it for me:

diff --git a/ext_mod/lvgl/micropython.mk b/ext_mod/lvgl/micropython.mk
index a205501..cbb44be 100644
--- a/ext_mod/lvgl/micropython.mk
+++ b/ext_mod/lvgl/micropython.mk
@@ -15,7 +15,8 @@ CFLAGS_USERMOD += -I$(LVGL_DIR)
 CFLAGS_USERMOD += -I$(LIB_DIR)
 
 
-ifdef $(LV_CFLAGS)
+ifdef LV_CFLAGS
     CFLAGS_USERMOD += $(LV_CFLAGS)
 endif

Thanks again for this awesome project!