Fix compiler warning: zero-initialize tm struct in DateTime::getTmUtc() - #754

What changes were made

Replaced tm result = {0}; with value-initialization tm result = {}; in FIX::DateTime::getTmUtc() inside src/C++/FieldTypes.h.

Why the changes were necessary

Compiling QuickFIX with compiler warnings enabled (-Wextra / -Wall) triggers a -Wmissing-field-initializers warning on struct tm:

src/C++/FieldTypes.h:231:19: warning: missing initializer for member ‘tm::tm_min’ [-Wmissing-field-initializers] 
src/C++/FieldTypes.h:231:19: warning: missing initializer for member ‘tm::tm_hour’ [-Wmissing-field-initializers] 
src/C++/FieldTypes.h:231:19: warning: missing initializer for member ‘tm::tm_gmtoff’ [-Wmissing-field-initializers] 
src/C++/FieldTypes.h:231:19: warning: missing initializer for member ‘tm::tm_zone’ [-Wmissing-field-initializers]

Using {0} only explicitly initializes the first struct member (tm_sec), causing GCC/Clang to warn about missing initializers for POSIX extension members (such as tm_gmtoff and tm_zone).

Updating to tm result = {}; performs standard C++ value-initialization across all fields of struct tm without triggering -Wmissing-field-initializers warnings.

Related Issues

Fixes #753