braces suggested : {} (was ACTS-464)

Original author Andreas Salzburger @asalzbur

<188/235> Building CXX object external/acts-core/Tests/Utilities/CMakeFiles/BFieldMapUtilsTests.dir/BFieldMapUtilsTests.cpp.o ../external/acts-core/Tests/Utilities/BFieldMapUtilsTests.cpp:91:45: warning: suggest braces around initialization of subobject <-Wmissing-braces> = bField_rz.at(localToGlobalBin_rz({0, 0}, {rPos.size(), zPos.size()})); ^~~~ { } ../external/acts-core/Tests/Utilities/BFieldMapUtilsTests.cpp:91:53: warning: suggest braces around initialization of subobject <-Wmissing-braces> = bField_rz.at(localToGlobalBin_rz({0, 0}, {rPos.size(), zPos.size()}));