In MuPdf-1.20.0 the search API, in particular the function
fz_search_stext_page was extended by an additional parameter.

Because of that, building zathura-pdf-mupdf against MuPDF-1.20.0 fails.

Therefore, I adapted the fz_search_stext_page call, adding NULL as the
new hit_mark parameter.

From this change on, >=MuPDF-1.20.0 is necessary to build
zathura-pdf-mupdf.