Automake expects `pkgconfigdir` to be present here, add it.

Upstream-status: Not presented upstream.

diff --git a/Makefile.am b/Makefile.am
index 03ed7e65..a05b022a 100644
--- a/Makefile.am
+++ b/Makefile.am
@@ -38,6 +38,7 @@ EXTRA_DIST = \
        libtpms.pc.in \
        autogen.sh
 
+pkgconfigdir = $(libdir)/pkgconfig
 pkgconfig_DATA = libtpms.pc
 
 DISTCHECK_CONFIGURE_FLAGS = --with-openssl --with-tpm2