Fix install when using a separated build directory

This fixes the following error at 'make install' time when
using a separated build directory (out-of-tree build):

CMake Error at cmake_install.cmake:50 (file):
  file INSTALL cannot find "/build/libilbc/src/libilbc/ilbc_export.h": No
  such file or directory.
make: *** [Makefile:105: install] Error 1
make: Leaving directory '/build/libilbc/src/build'

How to reproduce:

$ git clone --recursive https://github.com/TimothyGu/libilbc.git
$ cmake -B build -S libilbc
$ make -C build
$ make -C build install

Fixes TimothyGu#28

CMakeLists.txt

@@ -323,7 +323,7 @@ add_custom_target(ilbc_test-sample
 
 install(FILES ${CMAKE_CURRENT_BINARY_DIR}/libilbc.pc
         DESTINATION ${CMAKE_INSTALL_LIBDIR}/pkgconfig)
-install(FILES ilbc.h ilbc_export.h
+install(FILES ilbc.h ${CMAKE_CURRENT_BINARY_DIR}/ilbc_export.h
         DESTINATION ${CMAKE_INSTALL_INCLUDEDIR})
 install(FILES CONTRIBUTING.md NEWS.md README.md
         DESTINATION ${CMAKE_INSTALL_DOCDIR})