Drop the Windows-only isnt.o from UNRAR_OBJS.

vfs_rar's Makefile predates unrar's platform split and still lists isnt.o
(Windows version detection -- DWORD/OSVERSIONINFO/GetVersionEx), which no
longer compiles on Linux with modern unrar (7.2.6). unrar's own Linux
makefile excludes it as well.
--- a/Makefile
+++ b/Makefile
@@ -41,7 +41,6 @@
 	      arcread.o \
 	      unicode.o \
 	      system.o \
-	      isnt.o \
 	      crypt.o \
 	      crc.o \
 	      rawread.o \