libc: fix build in C23 mode

GCC 15 defaults to C23, in which bool, true, and false are keywords.

Signed-off-by: Yaakov Selkowitz <yselkowi@redhat.com>
Message-ID: <20250419114403.879376-1-yselkowi@redhat.com>
Tested-by: Vaibhav Jain <vaibhav@linux.ibm.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>