duplicate symbol in test_interpolation.cpp

[shimwell]

Bug Description

When compiling openmc with emcmake and with testing enabled an error with a duplicate symbol in test_interpolation.cpp was noticed.

wasm-ld: error: duplicate symbol: openmc::interpolate(gsl::span, gsl::span, double, openmc::Interpolation)

defined in CMakeFiles/test_interpolate.dir/test_interpolate.cpp.o
defined in ../../lib/libopenmc.a(filter_energyfunc.cpp.o)

Steps to Reproduce

See this comment for reproduce steps shimwell/openmc-online-demo#1 (comment)

Environment

ubuntu, emsdk:3.1.49, openmc develop branch