Add readwav.hxx to HEADERS and readwav.cxx to SOURCES in all cases

Previously, these two files were only added when SG was built with
-DUSE_AEONWAVE:BOOL=OFF, because the SG code in use when USE_AEONWAVE is
set to ON doesn't need readwav.hxx nor readwav.cxx. However, readwav.hxx
and readwav.cxx are not only used in SG, but also in FG (at least by
flightgear/src/Sound/VoiceSynthesizer.cxx), and this was causing a build
failure when SG's USE_AEONWAVE and FG's ENABLE_FLITE flags were both set
to ON:

  https://forum.flightgear.org/viewtopic.php?f=45&t=35672#p346633

and

  https://sourceforge.net/p/flightgear/mailman/message/36645567/

This commit should fix this build failure.

simgear/sound/CMakeLists.txt

@@ -6,6 +6,7 @@ set(HEADERS
     xmlsound.hxx
     soundmgr.hxx
     filters.hxx
+    readwav.hxx
     )
     
 set(SOURCES 
@@ -13,6 +14,7 @@ set(SOURCES
     sample_group.cxx
     xmlsound.cxx
     filters.cxx
+    readwav.cxx
     )
 
 if (USE_AEONWAVE)
@@ -20,13 +22,9 @@ if (USE_AEONWAVE)
         soundmgr_aeonwave.cxx
         )
 else()
-    set(HEADERS ${HEADERS}
-        readwav.hxx
-        )
     set(SOURCES ${SOURCES}
         soundmgr_openal.cxx
         soundmgr_openal_private.hxx
-        readwav.cxx
         )
 endif()