[libc] Remove warning about unused variable

[michaelrj]

LGTM

libc/src/assert/__assert_fail.cpp

 #include "src/assert/__assert_fail.h"
 #include "src/stdlib/abort.h"
 
 namespace __llvm_libc {
 
 LLVM_LIBC_FUNCTION(void, __assert_fail,
                    (const char *assertion, const char *file, unsigned line,
                     const char *function)) {
+  (void)line; // Suppress warning as long as line is unused.
   write_to_stderr(file);
   write_to_stderr(": Assertion failed: '");
   write_to_stderr(assertion);
   write_to_stderr("' in function: '");
   write_to_stderr(function);
   write_to_stderr("'\n");
   __llvm_libc::abort();
 }
 
 } // namespace __llvm_libc