lib: rsa: struct udevice build warning

Avoid build warnings observed with gcc 10.2

In file included from lib/rsa/rsa-keyprop.c:16:
include/u-boot/rsa-mod-exp.h:65:24: warning:
‘struct udevice’ declared inside parameter list will not be visible
outside of this definition or declaration
   65 | int rsa_mod_exp(struct udevice *dev, const uint8_t *sig,
      | uint32_t sig_len,
      |                        ^~~~~~~
include/u-boot/rsa-mod-exp.h:96:24: warning:
‘struct udevice’ declared inside parameter list will not be visible
outside of this definition or declaration
   96 |  int (*mod_exp)(struct udevice *dev, const uint8_t *sig,
      |

by defining struct udevice.

Fixes: 401d1c4f5d ("common: Drop asm/global_data.h from common header")
Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Reviewed-by: Simon Glass <sjg@chromium.org>

include/u-boot/rsa-mod-exp.h

@@ -9,6 +9,8 @@
 #include <errno.h>
 #include <image.h>
 
+struct udevice;
+
 /**
  * struct key_prop - holder for a public key properties
  *