build:  Use AX for pkg config

Use an autoconf archive macro for pkg config so autoconf can fill
out Requires.private in phosphor-dbus-interfaces.pc.in.

Change-Id: Ica803a0f758871b192502be5c2cdc6e166337df2
Signed-off-by: Brad Bishop <bradleyb@fuzziesquirrel.com>
diff --git a/phosphor-dbus-interfaces.pc.in b/phosphor-dbus-interfaces.pc.in
index 3240872..c669aa7 100644
--- a/phosphor-dbus-interfaces.pc.in
+++ b/phosphor-dbus-interfaces.pc.in
@@ -7,5 +7,6 @@
 Description: Generated sdbusplus bindings for phosphor-dbus-interfaces.
 URL: https://github.com/openbmc/phosphor-dbus-interfaces
 Version: @VERSION@
+Requires.private: @AX_PACKAGE_REQUIRES_PRIVATE@
 Libs: -L@libdir@ -lphosphor_dbus
 Cflags: -I@includedir@