infra: install capi header file only its binding option is enabled.

decouple the capi header from the default headers by replacing it in capi folder.
now it's also installed optionally.

inc/meson.build

@@ -1,7 +1,3 @@
 header_files = ['thorvg.h']
 
-if get_option('bindings').contains('capi') == true
-    header_files += ['thorvg_capi.h']
-endif
-
 install_headers(header_files)

src/bindings/capi/meson.build

@@ -6,3 +6,6 @@ subbinding_dep += [declare_dependency(
     include_directories : include_directories('.'),
     sources : source_file
 )]
+
+install_headers('thorvg_capi.h')
+headers += include_directories('.')