doc: add libgnutls28-dev to build dependencies

mkeficapsule requires package libgnutls28-dev for building

Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
diff --git a/doc/build/gcc.rst b/doc/build/gcc.rst
index b883cf7..470a7aa 100644
--- a/doc/build/gcc.rst
+++ b/doc/build/gcc.rst
@@ -25,11 +25,12 @@
 
     sudo apt-get install bc bison build-essential coccinelle \
       device-tree-compiler dfu-util efitools flex gdisk graphviz imagemagick \
-      liblz4-tool libguestfs-tools libncurses-dev libpython3-dev libsdl2-dev \
-      libssl-dev lz4 lzma lzma-alone openssl pkg-config python3 \
-      python3-coverage python3-pkg-resources python3-pycryptodome \
-      python3-pyelftools python3-pytest python3-sphinxcontrib.apidoc \
-      python3-sphinx-rtd-theme python3-virtualenv swig
+      liblz4-tool libgnutls28-dev libguestfs-tools libncurses-dev \
+      libpython3-dev libsdl2-dev libssl-dev lz4 lzma lzma-alone openssl \
+      pkg-config python3 python3-coverage python3-pkg-resources \
+      python3-pycryptodome python3-pyelftools python3-pytest \
+      python3-sphinxcontrib.apidoc python3-sphinx-rtd-theme python3-virtualenv \
+      swig
 
 SUSE based
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