cmake: Reorder subdirectories

This change ensures that both summaries—secp256k1's and Bitcoin Core's—
are displayed without interleaving of the `find_package(Doxygen)` output
from the `doc` subdirectory.

CMakeLists.txt

@@ -579,10 +579,11 @@ if(DEFINED ENV{LDFLAGS})
 endif()
 
 set(CMAKE_SKIP_RPATH TRUE)
-add_subdirectory(src)
 add_subdirectory(test)
 add_subdirectory(doc)
 
+add_subdirectory(src)
+
 include(cmake/tests.cmake)
 
 include(Maintenance)