Avoid redundant redeclaration of GetWarnings(const string&)

Summary:
std::string GetWarnings(const std::string& strFor) is declared in
warnings.h and validation.h, and defined in warnings.cpp.

Backport of core PR10864

Test Plan: Run with the -testsafemode=1 argument and check the warning appears

Reviewers: #bitcoin_abc, deadalnix, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Subscribers: teamcity, schancel

Differential Revision: https://reviews.bitcoinabc.org/D2201