[committed] correct a built-in declaration in gcc.dg/torture/pr54261-1.c

[Martin Sebor]

Jeff's build-bot reported warnings in the test as a result of the recent
tightening up of the -Wbuiltin-declaration-mismatch checking. The commit
at the following link adjusts the test to avoid the warning:

   https://gcc.gnu.org/pipermail/gcc-cvs/2020-March/271705.html

Tested on x86_64-linux both with and without the built-in declaration.

Martin