2.1.1 release missing pkgconfig .pc for libsdsl.a

[donpellegrino]

The 2.1.1 release only builds libdivsufsort64.pc and libdivsufsort.pc in lib/pkgconfig. This results in 'undefined reference to `sdsl::ram_fs_initializer::ram_fs_initializer()'" when linking with just libdivsufsort64.pc.

It seems that there should be a sdsl-lite.pc pkgconfig file to link with all necessary libraries for users of sdsl-lite.