[cpp-rest-sdk] remove U macro definition from public headers

[aminya]

Cpprest has a public U macro definition that causes build errors in libraries as U is a common name for template parameters. This PR fixes the issue by turning it off.

microsoft/cpprestsdk@411a109/Release/include/cpprest/details/basic_types.h#L87

Related to microsoft/cpprestsdk#1805 and fmtlib/fmt#4180

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[wing328]

At global scope:
cc1plus: note: unrecognized command-line option ‘-Wno-unused-lambda-capture’ may have been intended to silence earlier diagnostics
[ 52%] Building CXX object CMakeFiles/CppRestPetstoreClient.dir/src/model/SchemaWithSet.cpp.o
[ 56%] Building CXX object CMakeFiles/CppRestPetstoreClient.dir/src/model/SchemaWithSet_vaccinationBook.cpp.o
[ 60%] Building CXX object CMakeFiles/CppRestPetstoreClient.dir/src/model/Tag.cpp.o
[ 64%] Building CXX object CMakeFiles/CppRestPetstoreClient.dir/src/model/User.cpp.o
[ 68%] Building CXX object CMakeFiles/CppRestPetstoreClient.dir/src/model/Vaccine.cpp.o
[ 72%] Linking CXX static library libCppRestPetstoreClient.a

tests passed locally