build: Fix kernel static lib component install
Fixes the installation of the pkgconfig file and the static library when installing only the Kernel component.
This is a followup to fix #30835 and #30814, which were merged shortly after one another, but are interrelated. Can be tested with:
cmake -B build -DBUILD_SHARED_LIBS=OFF -DBUILD_KERNEL_LIB=ON
cmake --build build --target bitcoinkernel
cmake --install build --component Kernel