codeowners: Add build system to snippets and shields

Adds build system to be present on reviews for these folders

Signed-off-by: Jamie McCrae <jamie.mccrae@nordicsemi.no>

CODEOWNERS

@@ -56,6 +56,9 @@
 /boards/nordic/thingy91*                  @nrfconnect/ncs-co-boards @nrfconnect/ncs-cia
 /boards/shields/coverage_support/         @nrfconnect/ncs-low-level-test
 
+/boards/shields/                          @nrfconnect/ncs-co-build-system
+/boards/*/shields/                        @nrfconnect/ncs-co-build-system
+
 /boards/shields/pca63566/doc/*.rst        @nrfconnect/ncs-doc-leads
 
 # SUIT configuration files
@@ -635,6 +638,8 @@
 /snippets/wpa-supplicant-debug/           @krish2718 @sachinthegreen
 /snippets/zperf/                          @nrfconnect/ncs-protocols-serialization
 
+/snippets/                                @nrfconnect/ncs-co-build-system
+
 /snippets/nordic-bt-rpc/*.rst             @nrfconnect/ncs-doc-leads
 
 # Subsystems