Fix TTHREADINCDIR value for haiku OS

rtl/haiku/Makefile.fpc

@@ -57,7 +57,7 @@ SYSINITUNITS=si_c si_dllc
 UNITPREFIX=rtl
 DYNLIBS_DEPS_OS=$(DLUNIT)$(PPUEXT)
 DYNLIBSINCDIR=$(UNIXINC)
-TTHREADINCDIR=$(WINDIR)
+TTHREADINCDIR=$(UNIXINC)
 
 ifeq ($(ARCH),i386)
 LOADERS=cprt0 dllcprt0