[C++] Extra semicolon warning from ARROW_SUPPRESS_DEPRECATION_WARNING macro with -Wpedantic

[thisisnic]

This shows up when building the R package:

/home/nic/arrow-dist/include/arrow/util/macros.h:160:35: warning: extra ‘;’ [-Wpedantic]
     160 |     _Pragma("GCC diagnostic push");          \
         |                                   ^
   /home/nic/arrow-dist/include/arrow/compute/api_vector.h:277:3: note: in expansion of macro ‘ARROW_SUPPRESS_DEPRECATION_WARNING’
     277 |   ARROW_SUPPRESS_DEPRECATION_WARNING
         |   ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

The ARROW_SUPPRESS_DEPRECATION_WARNING macro for GCC has a trailing semicolon after the first _Pragma directive:

arrow/cpp/src/arrow/util/macros.h

Lines 159 to 161 in 83c185b

	# define ARROW_SUPPRESS_DEPRECATION_WARNING \
	_Pragma("GCC diagnostic push"); \
	_Pragma("GCC diagnostic ignored \"-Wdeprecated-declarations\"")

Since _Pragma is a complete directive, the semicolon becomes an empty statement, triggering -Wpedantic warnings wherever the macro is used (e.g. api_vector.h:134):

/arrow/util/macros.h:160:35: warning: extra ';' [-Wpedantic]

The same issue exists on the clang branch (line 155).

Removing the trailing semicolons from the _Pragma lines in the macro definition would fix it.

Authored by Claude, reviewed by thisisnic