dependencies: pureos-trisquel: Add graphicsmagick for the website.

Without that fix we have the following issue when building a release:

    ROM image release archives available at release/roms/

    checking for a BSD-compatible install... /usr/bin/install -c
    checking whether build environment is sane... yes
    checking for a race-free mkdir -p... /usr/bin/mkdir -p
    checking for gawk... gawk
    checking whether make sets $(MAKE)... yes
    checking whether make supports nested variables... yes
    checking for awk... awk
    [...]
    checking for gm... no

This issue was introduced by the commit
08b9e449e9 ("Add a minimal GNU Boot
manual.").

Signed-off-by: Denis 'GNUtoo' Carikli <GNUtoo@cyberdimension.org>
Tested-by: Adrien 'neox' Bourmault <neox@gnu.org>
Acked-by: Adrien 'neox' Bourmault <neox@gnu.org>

resources/dependencies/pureos-trisquel

@@ -130,8 +130,9 @@ install_packages gawk git grep lzip make sed tar xz-utils # u-boot-libre
 
 # For building the website tarball
 install_packages \
-    pandoc \
-    graphviz
+    graphicsmagick \
+    graphviz \
+    pandoc
 
 if [ -n "${to_install}" ] ; then
     apt install -y ${to_install}