[CMake] Fix dropped dependency in install-llvm-headers

This dependency was accidentally dropped in r319480, causing
install-distribution and install-llvm-headers to install an incomplete
set of headers (the generated Intrinsics and Attributes would be
missing).

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@334452 91177308-0d34-0410-b5e6-96231b3b80d8

CMakeLists.txt

@@ -985,6 +985,7 @@ if (NOT LLVM_INSTALL_TOOLCHAIN_ONLY)
 
   if (NOT CMAKE_CONFIGURATION_TYPES)
     add_llvm_install_targets(install-llvm-headers
+                             DEPENDS llvm-headers
                              COMPONENT llvm-headers)
   endif()
 endif()