Fix path to patterns directory for out-of-source builds

[zmajeed]

make install fails for out-of-source builds with this error

make  install-data-hook
make[3]: Entering directory 'ugrep/build'
mkdir -p ugrep/install/share/ugrep && \
          cp -rf patterns ugrep/install/share/ugrep/
cp: cannot stat 'patterns': No such file or directory
make[3]: *** [Makefile:914: install-data-hook] Error 1
make[3]: Leaving directory 'ugrep/build'
make[2]: *** [Makefile:820: install-data-am] Error 2
make[2]: Leaving directory 'ugrep/build'
make[1]: *** [Makefile:773: install-am] Error 2
make[1]: Leaving directory 'ugrep/build'
make: *** [Makefile:464: install-recursive] Error 1

This PR prepends $(top_srcdir) to patterns in the source tree to form a correct path

Fixes #219