uart.c: fix build with musl libc

musl does not define CMSPAR on all archs, patch inspired by
http://git.alpinelinux.org/cgit/aports/plain/main/freerdp/musl-fix.patch

musl needs sys/select.h to provide fd_set.

Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>
Signed-off-by: Brendan Le Foll <brendan.le.foll@intel.com>

src/uart/uart.c

@@ -29,10 +29,15 @@
 #include <unistd.h>
 #include <string.h>
 #include <termios.h>
+#include <sys/select.h>
 
 #include "uart.h"
 #include "mraa_internal.h"
 
+#ifndef CMSPAR
+#define CMSPAR   010000000000
+#endif
+
 // This function takes an unsigned int and converts it to a B* speed_t
 // that can be used with linux/posix termios
 static speed_t