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
diff --git a/libunwindstack/include/unwindstack/Regs.h b/libunwindstack/include/unwindstack/Regs.h
index 878ced3..1c2a81c 100644
--- a/libunwindstack/include/unwindstack/Regs.h
+++ b/libunwindstack/include/unwindstack/Regs.h
@@ -18,6 +18,7 @@
#define _LIBUNWINDSTACK_REGS_H
#include <stdint.h>
+#include <unistd.h>
#include <functional>
#include <string>