rtl-sdr: fix version in librtlsdr.pc

Before the fix:
```sh
$ grep Version result/lib/pkgconfig/librtlsdr.pc
Version: v0.8.x-xxx-xunknown
```
It causes problem while building projects that use librtlsdr:
```sh
-- Checking for module 'librtlsdr>=0.5.4'
-- Requested 'librtlsdr >= 0.5.4' but version of RTL-SDR Library is v0.8.x-xxx-xunknown
```

After the fix:
```sh
$ grep Version result/lib/pkgconfig/librtlsdr.pc
Version: 0.8.0
```

pkgs/applications/radio/rtl-sdr/default.nix

···
 
   postPatch = ''
     substituteInPlace CMakeLists.txt \
-      --replace '/etc/udev/rules.d' "$out/etc/udev/rules.d"
+      --replace '/etc/udev/rules.d' "$out/etc/udev/rules.d" \
+      --replace "VERSION_INFO_PATCH_VERSION git" "VERSION_INFO_PATCH_VERSION ${lib.versions.patch version}"
 
     substituteInPlace rtl-sdr.rules \
       --replace 'MODE:="0666"' 'ENV{ID_SOFTWARE_RADIO}="1", MODE="0660", GROUP="plugdev"'