[xen master] xen: fix compilation issue of serial.c

[patchbot@xxxxxxx]

commit c6dbc6df5266e10131fd04ad68ba5118fc257802
Author:     Oleksii Kurochko <oleksii.kurochko@xxxxxxxxx>
AuthorDate: Mon Dec 18 15:19:01 2023 +0100
Commit:     Jan Beulich <jbeulich@xxxxxxxx>
CommitDate: Mon Dec 18 15:19:01 2023 +0100

    xen: fix compilation issue of serial.c
    
    The following issue occurs on RISC-V platforms:
    drivers/char/serial.c: In function 'serial_tx_interrupt':
    drivers/char/serial.c:88:9: error: implicit declaration of function 
'cpu_relax' [-Werror=implicit-function-declaration]
       88 |         cpu_relax();
    
    cpu_relax() is defined in <asm/processor.h> so it was added
    an inclusion of the header to serial.c.
    
    Signed-off-by: Oleksii Kurochko <oleksii.kurochko@xxxxxxxxx>
    Acked-by: Jan Beulich <jbeulich@xxxxxxxx>
---
 xen/drivers/char/serial.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/xen/drivers/char/serial.c b/xen/drivers/char/serial.c
index 6d792f06dd..324024c29a 100644
--- a/xen/drivers/char/serial.c
+++ b/xen/drivers/char/serial.c
@@ -13,6 +13,8 @@
 #include <xen/serial.h>
 #include <xen/cache.h>
 
+#include <asm/processor.h>
+
 /* Never drop characters, even if the async transmit buffer fills. */
 /* #define SERIAL_NEVER_DROP_CHARS 1 */
 
--
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