Add unistd.h to includes in Regs.h

This is required to use pid_t, but by luck nobody noticed its absence
until the Fuchsia build.

Test: Code now builds inside Fuchsia

libunwindstack/include/unwindstack/Regs.h

@@ -18,6 +18,7 @@
 #define _LIBUNWINDSTACK_REGS_H
 
 #include <stdint.h>
+#include <unistd.h>
 
 #include <functional>
 #include <string>