Fix compiler warning

The same function iasecc_sm_external_authentication() was declared in
two different .h files.

In file included from ../../src/libopensc/iasecc.h:27:0,
                 from sm-card-iasecc.c:44:
../../src/libopensc/iasecc-sdo.h:324:5: warning: redundant redeclaration of `iasecc_sm_external_authentication' [-Wredundant-decls]
In file included from ../../src/libopensc/opensc.h:44:0,
                 from sm-card-iasecc.c:40:
../../src/libopensc/sm.h:352:5: note: previous declaration of `iasecc_sm_external_authentication' was here