package/kmsxx: fix gcc-10.x compile

package/kmsxx: fix gcc-10.x compile

Backport upstream commit ([1]) adding missing string include.

Fixes:
  - http://autobuild.buildroot.net/results/53a5f023ae40db18f45ebe7578962914c2d22a44

  In file included from .../build/kmsxx-cb0786049f960f2bd383617151b01318e02e9ff9/kms++/inc/kms++/omap/omapcard.h:3,
                   from .../build/kmsxx-cb0786049f960f2bd383617151b01318e02e9ff9/kms++/src/omap/omapcard.cpp:2:
  .../build/kmsxx-cb0786049f960f2bd383617151b01318e02e9ff9/kms++/inc/kms++/card.h:17:18: error: 'string' in namespace 'std' does not name a type
     17 |  Card(const std::string& device);
        |                  ^~~~~~

[1] https://github.com/tomba/kmsxx/commit/b53f9d383c9189a897c44cd88a8fc1b871fdc8a2.patch

Signed-off-by: Peter Seiderer <ps.report@gmx.net>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>