travis.yml: Add missing 'flex', 'bison' packages to 'GCC (user)' job

Since commit fd8171fe("target/hexagon: import lexer for idef-parser") the
hexagon target uses 'flex', 'bison' to generate idef-parser. However default
travis builder image for 'focal' may not have these pre-installed, consequently
following error is seen with travis when trying to execute the 'GCC (user)' job
that also tries to build hexagon user binary:

<snip>
export CONFIG="--disable-containers --disable-system"
<snip>
 Program flex found: NO

../target/hexagon/meson.build:179:4: ERROR: Program 'flex' not found or not
executable
<snip>

Fix this by explicitly add 'flex' and 'bison' to the list of addon apt-packages
for the 'GCC (user)' job.

Signed-off-by: Vaibhav Jain <vaibhav@linux.ibm.com>
Message-Id: <20230417162354.186678-1-vaibhav@linux.ibm.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Signed-off-by: Thomas Huth <thuth@redhat.com>

.travis.yml

@@ -237,6 +237,8 @@ jobs:
           - libglib2.0-dev
           - libgnutls28-dev
           - ninja-build
+          - flex
+          - bison
       env:
         - CONFIG="--disable-containers --disable-system"