Kernel POSIX: add stpcpy.

- Required by new GCC versions, which are able to optimize string
  appends using it.
- Fixes build of ACPICA for x86/gcc4.

src/system/kernel/lib/Jamfile

@@ -123,6 +123,7 @@ KernelMergeObject kernel_lib_posix.o :
 	strstr.c
 	strtok.c
 	strupr.c
+	stpcpy.c
 
 	: $(TARGET_KERNEL_PIC_CCFLAGS)
 ;