Fix two unqualified usages of size_t

[ybbat]

Problem:

• print_junit_summary uses unqualified size_t a couple of times, the cppm file does import std; which only guarantees std::size_t. Worked fine on Linux because libstdc++ provides ::size_t anyway but module failed to build on MinGW and presumably could fail on other targets too.

Solution:

• Add the std:: qualification

Issue: none, didn't seem worth raising an issue for this

Reviewers:
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