build:  Use AX for pkg config

Use an autoconf archive macro for pkg config so autoconf can fill
out Requires.private in sdbusplus.pc.in.

Change-Id: I62fd658b813ecde437756aefcee91ba42a3a94b2
Signed-off-by: Brad Bishop <bradleyb@fuzziesquirrel.com>
diff --git a/sdbusplus.pc.in b/sdbusplus.pc.in
index 6074514..2f7b4e2 100644
--- a/sdbusplus.pc.in
+++ b/sdbusplus.pc.in
@@ -7,6 +7,8 @@
 Description: C++ bindings for systemd dbus APIs
 URL: https://github.com/openbmc/sdbusplus
 Version: @VERSION@
+Requires: @AX_PACKAGE_REQUIRES@
+Requires.private: @AX_PACKAGE_REQUIRES_PRIVATE@
 Libs: -L@libdir@ -lsdbusplus
 Cflags: -I@includedir@