buildsys: The NSIS Windows build requires the documentation installed

buildsys: The NSIS Windows build requires the documentation installed

This fixes:

  $ make installer
  [...]
  (cd /tmp/qemu-nsis; \
           for i in qemu-system-*.exe; do \
             arch=${i%.exe}; \
             arch=${arch#qemu-system-}; \
             echo Section \"$arch\" Section_$arch; \
             echo SetOutPath \"\$INSTDIR\"; \
             echo File \"\${BINDIR}\\$i\"; \
             echo SectionEnd; \
           done \
          ) >/tmp/qemu-nsis/system-emulations.nsh
  makensis -V2 -NOCD \
                   \
                  -DCONFIG_GTK="y" \
                  -DBINDIR="/tmp/qemu-nsis" \
                   \
                  -DSRCDIR="/source/qemu" \
                  -DOUTFILE="qemu-setup-4.0.90.exe" \
                  -DDISPLAYVERSION="4.0.90" \
                  /source/qemu/qemu.nsi
  File: "/tmp/qemu-nsis\qemu-doc.html" -> no files found.
  Usage: File [/nonfatal] [/a] ([/r] [/x filespec [...]] filespec [...] |
     /oname=outfile one_file_only)
  Error in script "/source/qemu/qemu.nsi" on line 173 -- aborting creation process
  make: *** [Makefile:1080: qemu-setup-4.0.90.exe] Error 1

Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <20190715174817.18981-7-philmd@redhat.com>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>