rpm: ignore man3

During testing of RPM packages in MDEV-30203:
  file /usr/share/man/man3 from install of
  MariaDB-devel-11.0.1-1.el7_9.x86_64 conflicts with file from
  package filesystem-3.2-25.el7.x86_64

MariaDB is the first libmariadb to include man3 man pages
so make the changes here like what is done for man1 and man8.

cmake/cpack_rpm.cmake

@@ -155,6 +155,7 @@ SET(ignored
   "%ignore ${CMAKE_INSTALL_PREFIX}/share/doc"
   "%ignore ${CMAKE_INSTALL_PREFIX}/share/man"
   "%ignore ${CMAKE_INSTALL_PREFIX}/share/man/man1"
+  "%ignore ${CMAKE_INSTALL_PREFIX}/share/man/man3"
   "%ignore ${CMAKE_INSTALL_PREFIX}/share/man/man8"
   "%ignore ${CMAKE_INSTALL_PREFIX}/share/pkgconfig"
   )