fix(relibc): add missing P3-stdio-fseterr.patch to recipe

bison build was failing with:

  undefined reference to `__fseterr\'
  in function `rpl_fprintf'
  in function `rpl_vfprintf'

because the relibc recipe was missing P3-stdio-fseterr.patch, which
implements `__freadahead` and `__fseterr` in src/header/stdio/ext.rs
and adds the stdio_ext.h prototypes needed by gnulib (which m4, bison,
and flex all rely on).

The patch file was already present in local/patches/relibc/ with a
symlink at recipes/core/relibc/P3-stdio-fseterr.patch, but the recipe
never listed it. Without these symbols, any gnulib-using package that
links rpl_fprintf/rpl_vfprintf fails to link.

This is the same fix that was already applied to local/sources/relibc
fork at commit f45729d4 but not mirrored into the recipe patch chain
for upstream builds.

recipes/core/relibc/recipe.toml

@@ -62,6 +62,9 @@ patches = [
     "P3-stddef-reorder.patch",
     # ld.so: parse DT_RPATH in addition to DT_RUNPATH (RUNPATH takes precedence per gABI)
     "P3-ldso-rpath-support.patch",
+
+    # stdio_ext.h + __freadahead / __fseterr (m4/bison/flex gnulib compatibility)
+    "P3-stdio-fseterr.patch",
 ]
 
 [build]