Added back custom target (eos-headers) to make headers show up in IDEs

Let's see if there's ever a "modern CMake" solution for this without a custom target.
target_sources(eos INTERFACE ${HEADERS}) doesn't seem to be a proper solution.

CMakeLists.txt

@@ -112,7 +112,7 @@ target_include_directories(eos INTERFACE ${nanoflann_INCLUDE_DIR})
 target_include_directories(eos INTERFACE ${eigen3_nnls_INCLUDE_DIR})
 
 # Custom target for the library, to make the headers show up in IDEs:
-#add_custom_target(eos SOURCES ${HEADERS}) # name it eos-headers?
+add_custom_target(eos-headers SOURCES ${HEADERS})
 source_group(core REGULAR_EXPRESSION include/eos/core/*)
 source_group(morphablemodel REGULAR_EXPRESSION include/eos/morphablemodel/*)
 source_group(morphablemodel\\io REGULAR_EXPRESSION include/eos/morphablemodel/io/*)