ci:coverity: fix uboot-tools cov-build failure.

uboot-tools (pylibfdt) failed to compile with:
fatal error: Python.h: No such file or directory

Add python3-dev to the apt-get install step to provide the
missing header.

Reference: PPW-2121
Signed-off-by: Mouna Aloui <mouna.aloui@softathome.com>

.gitlab/coverity.yml

@@ -29,7 +29,7 @@ include:
     - if: '$CI_MERGE_REQUEST_TARGET_BRANCH_PROTECTED == "true" && $CI_MERGE_REQUEST_LABELS =~ /ci::allow-expensive-jobs/'
   before_script:
     - export BRIDGE_COVERITY_BUILD_COMMAND="make -j $(nproc)"
-    - sudo apt-get -qq update && sudo apt-get install -y curl file unzip coccinelle lzop libssl-dev
+    - sudo apt-get -qq update && sudo apt-get install -y curl file unzip coccinelle lzop libssl-dev python3-dev
     - touch .build_failed
     - mkdir -p logs
     - set -o pipefail