doc: Add reportlab install info

[abmusse]

pip installing reportlab with python3.9 fails building _renderPM extension.

    In file included from /usr/include/sys/thread.h:41:0,
                     from /usr/include/sys/ptrace.h:28,
                     from /usr/include/sys/proc.h:42,
                     from /usr/include/sys/pri.h:43,
                     from /usr/include/sys/sched.h:38,
                     from /usr/include/sched.h:51,
                     from /usr/include/pthread.h:67,
                     from /QOpenSys/pkgs/include/python3.9/pythread.h:128,
                     from /QOpenSys/pkgs/include/python3.9/Python.h:131,
                     from src/rl_addons/renderPM/_renderPM.c:1:
    src/rl_addons/renderPM/libart_lgpl/art_pixbuf.h:38:41: error: expected ';', ',' or ')' before '.' token
     typedef void (*ArtDestroyNotify) (void *func_data, void *data);
                                             ^
    In file included from src/rl_addons/renderPM/libart_lgpl/libart.h:11:0,
                     from src/rl_addons/renderPM/_renderPM.c:11:
    src/rl_addons/renderPM/libart_lgpl/art_pixbuf.h:63:3: error: unknown type name 'ArtDestroyNotify'
       ArtDestroyNotify destroy;
       ^~~~~~~~~~~~~~~~
    src/rl_addons/renderPM/libart_lgpl/art_pixbuf.h:83:26: error: unknown type name 'ArtDestroyNotify'
            void *dfunc_data, ArtDestroyNotify dfunc);
                              ^~~~~~~~~~~~~~~~
    src/rl_addons/renderPM/libart_lgpl/art_pixbuf.h:87:27: error: unknown type name 'ArtDestroyNotify'
             void *dfunc_data, ArtDestroyNotify dfunc);

Workaround to resolve the build failure is to define _LINUX_SOURCE_COMPAT.