Un-break contrib install with llvm.

Un-break contrib install with llvm.

Apparently $(foreach ... $(call install_llvm_module,...)) doesn't work
too well without a blank line ending the install_llvm_module macro.
The previous coding hackishly dodged this problem with some parens,
but that's not really a good solution because make misunderstands
where the command boundaries are that way.

Discussion: https://postgr.es/m/20180428073935.GB1736@paquier.xyz