lib/{bitops,alloc_page}.h: Add missing headers

bitops.h uses the 'bool' and 'size_t' types, but doesn't include the
stddef.h and stdbool.h headers, where the types are defined. This can cause
the following error when compiling:

In file included from arm/new-test.c:9:
/path/to/kvm-unit-tests/lib/bitops.h:77:15: error: unknown type name 'bool'
   77 | static inline bool is_power_of_2(unsigned long n)
      |               ^~~~
/path/to/kvm-unit-tests/lib/bitops.h:82:38: error: unknown type name 'size_t'
   82 | static inline unsigned int get_order(size_t size)
      |                                      ^~~~~~
/path/to/kvm-unit-tests/lib/bitops.h:24:1: note: 'size_t' is defined in header '<stddef.h>'; did you forget to '#include <stddef.h>'?
   23 | #include <asm/bitops.h>
  +++ |+#include <stddef.h>
   24 |
make: *** [<builtin>: arm/new-test.o] Error 1

The same errors were observed when including alloc_page.h. Fix both files
by including stddef.h and stdbool.h.