[PR105324] libstdc++: testsuite: pr105324 requires FP from_char
The floating-point overloads of from_char are only declared if
_GLIBCXX_HAVE_USELOCALE is #defined as nonzero. That's exposed from
charconv as __cpp_lib_to_chars >= 201611L, so guard the test body with
that.
for libstdc++-v3/ChangeLog
PR c++/105324
* testsuite/20_util/from_chars/pr105324.cc: Guard test body
with conditional for floating-point overloads of from_char.
(cherry picked from commit
25389f3de489c25a6983db96428a6bf06aedc829)