pkg-config: Add missing description field

The old school `pkg-config` utility parses .pc files fine if they are
lacking description. However, using `pkgconf` will silently fail to find
the .pc file.

Change-Id: I690614de15c7d095bb4066ba4329045c86e4ee0e
Signed-off-by: William A. Kennington III <wak@google.com>
diff --git a/src/sdeventplus.pc.in b/src/sdeventplus.pc.in
index 3d67d40..91f946c 100644
--- a/src/sdeventplus.pc.in
+++ b/src/sdeventplus.pc.in
@@ -4,6 +4,7 @@
 includedir=@includedir@
 
 Name: sdeventplus
+Description: C++ systemd event wrapper
 Version: @VERSION@
 Requires: @AX_PACKAGE_REQUIRES@
 Requires.private: @AX_PACKAGE_REQUIRES_PRIVATE@