Add missing include in rwcollection.h

Summary: It is required to compile on win32

Test Plan:

cmake -GNinja .. -DCMAKE_TOOLCHAIN_FILE=../cmake/platforms/Win32.cmake -DBUILD_BITCOIN_SEEDER=Off
ninja all src/test/test_bitcoin.exe

Reviewers: #bitcoin_abc, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Subscribers: teamcity

Differential Revision: https://reviews.bitcoinabc.org/D2029