add BR for gtk-doc to fix build failure in Fedora Rawhide
    
    [...]
    + autoreconf -fiv
    autoreconf: export WARNINGS=
    autoreconf: Entering directory '.'
    autoreconf: configure.ac: not using Gettext
    autoreconf: running: aclocal --force -I m4 ${ACLOCAL_FLAGS}
    configure.ac:113: warning: The macro `AC_TRY_COMPILE' is obsolete.
    configure.ac:113: You should run autoupdate.
    ./lib/autoconf/general.m4:2846: AC_TRY_COMPILE is expanded from...
    m4/as-compiler-flag.m4:13: AS_COMPILER_FLAG is expanded from...
    configure.ac:113: the top level
    configure.ac:366: warning: The macro `AC_HELP_STRING' is obsolete.
    configure.ac:366: You should run autoupdate.
    ./lib/autoconf/general.m4:203: AC_HELP_STRING is expanded from...
    configure.ac:366: the top level
    configure.ac:113: warning: The macro `AC_TRY_COMPILE' is obsolete.
    configure.ac:113: You should run autoupdate.
    ./lib/autoconf/general.m4:2846: AC_TRY_COMPILE is expanded from...
    m4/as-compiler-flag.m4:13: AS_COMPILER_FLAG is expanded from...
    configure.ac:113: the top level
    configure.ac:366: warning: The macro `AC_HELP_STRING' is obsolete.
    configure.ac:366: You should run autoupdate.
    ./lib/autoconf/general.m4:203: AC_HELP_STRING is expanded from...
    configure.ac:366: the top level
    autoreconf: configure.ac: tracing
    autoreconf: running: libtoolize --copy --force
    libtoolize: putting auxiliary files in '.'.
    libtoolize: copying file './ltmain.sh'
    libtoolize: putting macros in AC_CONFIG_MACRO_DIRS, 'm4'.
    libtoolize: copying file 'm4/libtool.m4'
    libtoolize: copying file 'm4/ltoptions.m4'
    libtoolize: copying file 'm4/ltsugar.m4'
    libtoolize: copying file 'm4/ltversion.m4'
    libtoolize: copying file 'm4/lt~obsolete.m4'
    autoreconf: configure.ac: not using Intltool
    autoreconf: running: gtkdocize --copy
    Can't exec "gtkdocize": No such file or directory at /usr/share/autoconf/Autom4te/FileUtils.pm line 293.
    autoreconf: error: gtkdocize failed with exit status: 2
    RPM build errors:
    error: Bad exit status from /var/tmp/rpm-tmp.2ec84D (%prep)