Add apps/shared include dir to specific targets. (#84)

Add the apps/shared include directory only to the targets that need it
by using target_include_directories(). include_directories(apps/shared)
would add the apps/shared include directory to all targets.

CMakeLists.txt

diff --git a/CMakeLists.txt b/CMakeLists.txt
index fb27bb4..240c4d1 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -269,7 +269,6 @@
 
 option(AVIF_BUILD_APPS "Build avif apps." OFF)
 if(AVIF_BUILD_APPS)
-    include_directories(apps/shared)
     add_executable(avifenc
         apps/avifenc.c
         apps/shared/y4m.c
@@ -279,6 +278,7 @@
         set_target_properties(avifenc PROPERTIES LINKER_LANGUAGE "CXX")
     endif()
     target_link_libraries(avifenc avif ${AVIF_PLATFORM_LIBRARIES})
+    target_include_directories(avifenc PRIVATE apps/shared)
     add_executable(avifdec
         apps/avifdec.c
         apps/shared/y4m.c
@@ -288,6 +288,7 @@
         set_target_properties(avifdec PROPERTIES LINKER_LANGUAGE "CXX")
     endif()
     target_link_libraries(avifdec avif ${AVIF_PLATFORM_LIBRARIES})
+    target_include_directories(avifdec PRIVATE apps/shared)
 
     if(NOT SKIP_INSTALL_APPS AND NOT SKIP_INSTALL_ALL)
         install(TARGETS avifenc avifdec
@@ -300,7 +301,6 @@
 
 option(AVIF_BUILD_TESTS "Build avif tests." OFF)
 if(AVIF_BUILD_TESTS)
-    include_directories(apps/shared)
     add_executable(aviftest
         apps/shared/y4m.c
         tests/aviftest.c
@@ -312,6 +312,7 @@
         set_target_properties(aviftest PROPERTIES LINKER_LANGUAGE "CXX")
     endif()
     target_link_libraries(aviftest avif ${AVIF_PLATFORM_LIBRARIES})
+    target_include_directories(aviftest PRIVATE apps/shared)
 
     add_executable(avifyuv
         tests/avifyuv.c