libudev: add "Libs.private: -lrt -pthread" to libudev.pc

This resolves a failure when linking cryptsetup.static against libudev.a.

```
libtool: link: x86_64-pc-linux-gnu-gcc -Wall -O2 -pipe -march=amdfam10 -static -O2 -o cryptsetup.static lib/utils_crypt.o lib/utils_loop.o lib/utils_io.o lib/utils_blkid.o src/utils_tools.o src/utils_password.o src/utils_luks2.o src/utils_blockdev.o src/cryptsetup.o -pthread -pthread  -Wl,--as-needed ./.libs/libcryptsetup.a -largon2 -lrt -ljson-c -lpopt -luuid -lblkid -lssl -lcrypto -lz -ldl -ldevmapper -lm -lpthread -ludev -pthread
/usr/lib/gcc/x86_64-pc-linux-gnu/11.1.0/../../../../x86_64-pc-linux-gnu/bin/ld: /usr/lib/gcc/x86_64-pc-linux-gnu/11.1.0/../../../../lib64/libudev.a(src_libsystemd_sd-daemon_sd-daemon.c.o): in function `sd_is_mq':
(.text.sd_is_mq+0x3a): undefined reference to `mq_getattr'
```