Finish converting GSL to absl::Span

One less dependency.

Change-Id: Iaac25a3c1312a21a8a2b77b8cfee2463bdb51196
Signed-off-by: Austin Schuh <austin.linux@gmail.com>
diff --git a/motors/print/uart.h b/motors/print/uart.h
index f4a61c8..7d81577 100644
--- a/motors/print/uart.h
+++ b/motors/print/uart.h
@@ -18,7 +18,7 @@
 
   void Initialize() override;
 
-  int WriteStdout(gsl::span<const char> buffer) override;
+  int WriteStdout(absl::Span<const char> buffer) override;
 
  private:
   teensy::InterruptBufferedUart stdout_uart_;