gnu: libbonobo: Build sequentially.

gnu: libbonobo: Build sequentially.

See
<https://berlin.guixsd.org/log/5mr1fyac5kyz41zwmnf4b0wm7vl42ryv-libbonobo-2.32.1>
for an example of a parallel build failure:

    CCLD   echo-client-2
    CCLD   bonobo-echo-2
  ld: libEcho.a(Bonobo_Sample_Echo-stubs.o): in function `Bonobo_Sample_Echo_echo':
  /tmp/guix-build-libbonobo-2.32.1.drv-0/libbonobo-2.32.1/samples/echo/Bonobo_Sample_Echo-stubs.c:12: undefined reference to `Bonobo_Sample_Echo__iinterface'
  ld: /tmp/guix-build-libbonobo-2.32.1.drv-0/libbonobo-2.32.1/samples/echo/Bonobo_Sample_Echo-stubs.c:12: undefined reference to `Bonobo_Sample_Echo__classid'
  ld: /tmp/guix-build-libbonobo-2.32.1.drv-0/libbonobo-2.32.1/samples/echo/Bonobo_Sample_Echo-stubs.c:12: undefined reference to `_ORBIT_skel_small_Bonobo_Sample_Echo_echo'
  collect2: error: ld returned 1 exit status
  make[3]: *** [Makefile:488: echo-client-2] Error 1

* gnu/packages/gnome.scm (libbonobo)[arguments]: Add #:parallel-build?.