hid_enabled: fix a build-error on docker / alpine

[holiman]

This PR fixes an error on docker/alpine builds.

Error:

7.675 In file included from ./hid_enabled.go:47:                                                                                                                                                                            
7.675 ./libusb/libusb/os/threads_posix.c:54:5: error: conflicting types for 'usbi_cond_timedwait'; have 'int(pthread_cond_t *, pthread_mutex_t *, const struct timeval *)'                                                  
7.675    54 | int usbi_cond_timedwait(pthread_cond_t *cond,                                                                                                                                                                 
7.675       |     ^~~~~~~~~~~~~~~~~~~
7.675 In file included from ./hid_enabled.go:44:
7.675 ./libusb/libusb/os/threads_posix.h:67:5: note: previous declaration of 'usbi_cond_timedwait' with type 'int(usbi_cond_t *, usbi_mutex_t *, const struct timeval *)' {aka 'int(pthread_cond_t *, pthread_mutex_t *, const struct timeval *)'}
7.675    67 | int usbi_cond_timedwait(usbi_cond_t *cond,

This PR also adds two docker-files, which can be used to verify that both gcc and musl can build it.

$ docker build . -f ./Dockerfile.libc -t test && docker run test
...
hid.Supported() true
...
$ docker build . -f ./Dockerfile.musl -t test && docker run test
...
hid.Supported() true

[karalabe]

SGTM