patch 8.1.1408: PFL_HIDDEN conflicts with system header file

Problem:    PFL_HIDDEN conflicts with system header file.
Solution:   Rename to POPF_HIDDEN.
diff --git a/src/vim.h b/src/vim.h
index a52ba5e..42a415a 100644
--- a/src/vim.h
+++ b/src/vim.h
@@ -614,8 +614,8 @@
 #define VALID_TOPLINE	0x80	// w_topline is valid (for cursor position)
 
 // Values for w_popup_flags.
-#define PFL_HIDDEN	1	// popup is not displayed
-#define PFL_REDRAWN	2	// popup was just redrawn
+#define POPF_HIDDEN	1	// popup is not displayed
+#define POPF_REDRAWN	2	// popup was just redrawn
 
 /*
  * Terminal highlighting attribute bits.