emulators/yuzu: unbreak with pkgconf after 72cb3d2eba79

CMake Error at externals/libusb/CMakeLists.txt:109 (add_library):

Cannot find source file:

  libusb/libusb/core.c

Tried extensions .c .C .c++ .cc .cpp .cxx .cu .mpp .m .M .mm .h .hh .h++
.hm .hpp .hxx .in .txx .f .F .for .f77 .f90 .f95 .f03 .ispc

CMake Error at externals/libusb/CMakeLists.txt:109 (add_library):

No SOURCES given to target: usb

PR: 256912
Submitted by: jkim (based on)

(cherry picked from commit b2a8fc817e26955c5c60f9bcc540a7d335aa41bc)