[CMAKE] Fix static linkage when building for Windows

Summary:
The `-static` flag is obviously a linker flag and not a compilation
flag. It used to work due to the `CMAKE_<C|CXX>_FLAGS` being passed at
link time, hiding the initial issue. The bug is revealed by D4947: while
the cross build still succeed, it is no longer static.

Test Plan:
Run the Gitian build for Windows. Run the `bitcoin-qt.exe` executable
and check it runs and does not complain about missing DLLs.

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D5023

src/CMakeLists.txt

@@ -66,7 +66,7 @@ if(${CMAKE_SYSTEM_NAME} MATCHES "Windows")
 	list(APPEND CMAKE_RC_FLAGS "-DWINDRES_PREPROC")
 
 	# Build all static so there is no dll file to distribute.
-	add_compiler_flags(-static)
+	add_linker_flags(-static)
 endif()
 
 if(${CMAKE_SYSTEM_NAME} MATCHES "Darwin")