Update qt/qvaluecombobox.cpp to use Qt5 connect syntax

Summary:
Updates to use Qt5 connect syntax over Qt4.
See https://wiki.qt.io/New_Signal_Slot_Syntax for details.

Partial backport of Core PR13529
https://github.com/bitcoin/bitcoin/pull/13529/commits/f78558f1e39198779bdb17e2b0e256fb99ad4b28?file-filters%5B%5D=.cpp#diff-50388e8625cebcce224abde550638dfc

Test Plan:

make check
./bitcoin-qt
Send

Increment and decrement the amount value and manually input a value into the amount box.

Reviewers: deadalnix, Fabien, jasonbcox, O1 Bitcoin ABC, #bitcoin_abc

Reviewed By: jasonbcox, O1 Bitcoin ABC, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D4320