Update OTAPI.hpp - Fixes MC compile error

This fixes the forward declaration error when compiling MC.

Error was:

compiling ../../src/core/applicationmc.cpp
../../src/core/applicationmc.cpp: In member function 'void MTApplicationMC::appStarting()':
../../src/core/applicationmc.cpp:54:21: error: invalid use of incomplete type 'class OTAPI_Exec'
     OTAPI_Wrap::It()->LoadWallet();
                     ^
In file included from ../../src/core/applicationmc.cpp:10:0:
/usr/local/include/opentxs/OTAPI.hpp:166:7: error: forward declaration of 'class OTAPI_Exec'
 class OTAPI_Exec;
       ^
make[2]: *** [debug/applicationmc.o] Error 1
make[2]: Leaving directory `/home/lucas/Projects/OpenTransactions/Moneychanger/build-moneychanger-Desktop_Qt_5_2_1_GCC_64bit-Debug/moneychanger-qt'
make[1]: Leaving directory `/home/lucas/Projects/OpenTransactions/Moneychanger/build-moneychanger-Desktop_Qt_5_2_1_GCC_64bit-Debug/moneychanger-qt'
make[1]: *** [debug] Error 2
make: *** [sub-moneychanger-qt-make_first-ordered] Error 2
06:45:49: The process "/usr/bin/make" exited with code 2.
Error while building/deploying project moneychanger (kit: Desktop Qt 5.2.1 GCC 64bit)
When executing step 'Make'
06:45:49: Elapsed time: 00:10.