pkg-config: add Requires.private: libcap

The two shared libraries for now link against libcap,
so add it to the Requires.private as it's a dependency

Fixes https://github.com/systemd/systemd/issues/36117

src/libsystemd/libsystemd.pc.in

@@ -18,3 +18,4 @@ URL: {{PROJECT_URL}}
 Version: {{PROJECT_VERSION}}
 Libs: -L${libdir} -lsystemd
 Cflags: -I${includedir}
+Requires.private: libcap

src/libudev/libudev.pc.in

@@ -18,3 +18,4 @@ Version: {{PROJECT_VERSION}}
 Libs: -L${libdir} -ludev
 Libs.private: -lrt -pthread
 Cflags: -I${includedir}
+Requires.private: libcap