chore(3p/nix): export library headers to include/ in output.

Change-Id: I87eb6e59782d720015d351d8829dc7b8688e01f2
Reviewed-on: https://cl.tvl.fyi/c/depot/+/543
Reviewed-by: tazjin <mail@tazj.in>

third_party/nix/src/libmain/CMakeLists.txt

@@ -4,10 +4,14 @@ set_property(TARGET nixmain PROPERTY CXX_STANDARD 17)
 include_directories(${PROJECT_BINARY_DIR}) # for config.h
 target_include_directories(nixmain PUBLIC "${nix_SOURCE_DIR}/src")
 
-target_sources(nixmain
-  PUBLIC
+set(HEADER_FILES
     common-args.hh
     shared.hh
+)
+
+target_sources(nixmain
+  PUBLIC
+    ${HEADER_FILES}
   PRIVATE
     common-args.cc
     shared.cc
@@ -22,4 +26,5 @@ target_link_libraries(nixmain
   glog
 )
 
+INSTALL(FILES ${HEADER_FILES} DESTINATION ${CMAKE_INSTALL_INCLUDEDIR}/nix/libmain)
 INSTALL(TARGETS nixmain DESTINATION lib)