fix pthread_attr_* implementations to match corrected prototypes

src/thread/pthread_attr_getdetachstate.c

 #include "pthread_impl.h"
 
-int pthread_attr_getdetachstate(pthread_attr_t *a, int *state)
+int pthread_attr_getdetachstate(const pthread_attr_t *a, int *state)
 {
 	*state = a->_a_detach;
 	return 0;

src/thread/pthread_attr_getguardsize.c

 #include "pthread_impl.h"
 
-int pthread_attr_getguardsize(pthread_attr_t *a, size_t *size)
+int pthread_attr_getguardsize(const pthread_attr_t *a, size_t *size)
 {
 	*size = a->_a_guardsize + DEFAULT_GUARD_SIZE;
 	return 0;

src/thread/pthread_attr_getscope.c

 #include "pthread_impl.h"
 
-int pthread_attr_getscope(pthread_attr_t *a, int *scope)
+int pthread_attr_getscope(const pthread_attr_t *a, int *scope)
 {
 	return 0;
 }

src/thread/pthread_attr_getstacksize.c

 #include "pthread_impl.h"
 
-int pthread_attr_getstacksize(pthread_attr_t *a, size_t *size)
+int pthread_attr_getstacksize(const pthread_attr_t *a, size_t *size)
 {
 	*size = a->_a_stacksize + DEFAULT_STACK_SIZE;
 	return 0;