gdbserver: include linux-arm-tdesc.h in linux-arm-tdesc.c

When building with -Wmissing-declarations, I get:

  CXX    linux-arm-tdesc.o
/binutils-gdb/gdb/gdbserver/linux-arm-tdesc.c:29:1: error: no previous declaration for 'const target_desc* arm_linux_read_description(arm_fp_type)' [-Werror=missing-declarations]
   29 | arm_linux_read_description (arm_fp_type fp_type)
      | ^~~~~~~~~~~~~~~~~~~~~~~~~~
/binutils-gdb/gdb/gdbserver/linux-arm-tdesc.c:49:1: error: no previous declaration for 'arm_fp_type arm_linux_get_tdesc_fp_type(const target_desc*)' [-Werror=missing-declarations]
   49 | arm_linux_get_tdesc_fp_type (const target_desc *tdesc)
      | ^~~~~~~~~~~~~~~~~~~~~~~~~~~

Include linux-arm-tdesc.h in linux-arm-tdesc.c to fix it.  And because
linux-arm-tdesc.h uses the arm_fp_type, it should include arch/arm.h as
well.

gdb/gdbserver/ChangeLog:

	* linux-arm-tdesc.c: Include linux-arm-tdesc.h.
	* linux-arm-tdesc.h: Include arch/arm.h.

gdb/gdbserver/ChangeLog

@@ -1,3 +1,8 @@
+2020-01-12  Simon Marchi  <simon.marchi@polymtl.ca>
+
+	* linux-arm-tdesc.c: Include linux-arm-tdesc.h.
+	* linux-arm-tdesc.h: Include arch/arm.h.
+
 2020-01-12  Simon Marchi  <simon.marchi@polymtl.ca>
 
 	* linux-aarch64-low.c (aarch64_write_goto_address): Make static.

gdb/gdbserver/linux-arm-tdesc.c

@@ -16,6 +16,9 @@
    along with this program.  If not, see <http://www.gnu.org/licenses/>.  */
 
 #include "server.h"
+
+#include "linux-arm-tdesc.h"
+
 #include "tdesc.h"
 #include "arch/arm.h"
 #include <inttypes.h>

gdb/gdbserver/linux-arm-tdesc.h

@@ -18,6 +18,8 @@
 #ifndef GDBSERVER_LINUX_ARM_TDESC_H
 #define GDBSERVER_LINUX_ARM_TDESC_H
 
+#include "arch/arm.h"
+
 /* Return the Arm target description with fp registers FP_TYPE.  */
 
 const target_desc * arm_linux_read_description (arm_fp_type fp_type);