Add pkgconfig

Generate pkgconfig for phosphor-ipmi-blobs so that the library can be
found by Meson.

When used as a dependency, phosphor-ipmi-blobs provides only a header
file and no libraries to link against, so the pkgconfig file has no
linker flags.

Tested: added phosphor-ipmi-blobs as a dependency in
phosphor-health-monitor's meson.build file and built successfully.

signed-off-by: Sui Chen <suichen@google.com>
Change-Id: I126c66dbd65ddbd99f921297d813e7a0f9981f22
diff --git a/Makefile.am b/Makefile.am
index 974f338..6bfa178 100644
--- a/Makefile.am
+++ b/Makefile.am
@@ -106,3 +106,5 @@
 	-flto
 
 SUBDIRS = . test
+
+pkgconfig_DATA = phosphor-ipmi-blobs.pc