ci: Install qtbase5-private-dev on Linux

List this dependency both under CI/install-dependencies-linux.sh, and
.github/workflows/main.yml.

.github/workflows/main.yml

@@ -419,6 +419,7 @@ jobs:
            pkg-config \
            python3-dev \
            qtbase5-dev \
+           qtbase5-private-dev \
            libqt5svg5-dev \
            swig \
            libcmocka-dev \

CI/install-dependencies-linux.sh

@@ -42,6 +42,7 @@ sudo apt-get install -y \
         pkg-config \
         python3-dev \
         qtbase5-dev \
+        qtbase5-private-dev \
         libqt5svg5-dev \
         swig \
         linux-generic \