Fix .PHONY target to match new installheaders target.


git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@148629 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/Makefile b/Makefile
index 9546857..79385b2 100644
--- a/Makefile
+++ b/Makefile
@@ -16,7 +16,7 @@
 endif
 INSTALL_DIR=$(DSTROOT)/$(INSTALL_PREFIX)
 
-.PHONY: help installsrc clean installhdrs do-installhdrs install
+.PHONY: help installsrc clean installheaders do-installhdrs install
 
 help::
 	echo Use make install DSTROOT=<destination>