[libc]: fix missing linux/error-number-macros.h (llvm#97101)

The headers installation previously missed platform-specific number
macros, and this fixes that. From discussion on LLVM discord with
`michaelrj` – ccing @michaelrj-google who hopefully is the same person 😅

N.B. I have tested this manually in my Linux Dockerfile and it appears
to do the right thing; I haven't done much more than that.

libc/include/CMakeLists.txt

@@ -223,6 +223,7 @@ add_gen_header(
   GEN_HDR errno.h
   DEPENDS
     .llvm-libc-macros.generic_error_number_macros
+    .llvm-libc-macros.error_number_macros
 )
 
 add_gen_header(