Fix 'main' function in 'gcc.dg/builtin-dynamic-object-size-pr120780.c'

Fix 'main' function in 'gcc.dg/builtin-dynamic-object-size-pr120780.c'

Fix-up for commit 72e85d46472716e670cbe6e967109473b8d12d38
"tree-optimization/120780: Support object size for containing objects".
'size_t sz' is unused here, and GCC/nvptx doesn't accept this:

    spawn -ignore SIGHUP [...]/nvptx-none-run ./builtin-dynamic-object-size-pr120780.exe
    error   : Prototype doesn't match for 'main' in 'input file 1 at offset 1924', first defined in 'input file 1 at offset 1924'
    nvptx-run: cuLinkAddData failed: unknown error (CUDA_ERROR_UNKNOWN, 999)
    FAIL: gcc.dg/builtin-dynamic-object-size-pr120780.c execution test

	gcc/testsuite/
	* gcc.dg/builtin-dynamic-object-size-pr120780.c: Fix 'main' function.