Use the system pcre2 header instead of the hardcoded bundled path.

Godot 4.8 moved the RegEx implementation out of modules/regex/ into
core/string/regex.cpp and, while converting the tree to IWYU-style includes
(paths relative to the repository root), hardcoded the bundled copy:

    #include <thirdparty/pcre2/src/pcre2.h>

That path is only reachable through core/SCsub, which prepends
"#thirdparty/pcre2/src/" to CPPPATH exclusively under `if env["builtin_pcre2"]`
(and only for env_thirdparty at that).  With builtin_pcre2=no,
platform/linuxbsd/detect.py resolves pcre2 via `pkg-config libpcre2-32`, which
on Gentoo emits just -lpcre2-32 and no -I, since the header is installed as
/usr/include/pcre2.h.  Compiling core/string/regex.cpp then fails with:

    fatal error: thirdparty/pcre2/src/pcre2.h: No such file or directory

regex.cpp already defines PCRE2_STATIC and PCRE2_CODE_UNIT_WIDTH 32 just above
the include, so pointing at the system header is all that is needed
(PCRE2_STATIC is inert on ELF platforms, it only drives __declspec on Windows).

This unconditional rewrite is safe here because the ebuild always unbundles
pcre2 (thirdparty/pcre2 is deleted in src_prepare and builtin_pcre2=no is
passed to scons), so the bundled path is never valid in this build.

--- a/core/string/regex.cpp
+++ b/core/string/regex.cpp
@@ -42,7 +42,7 @@
 #define PCRE2_CODE_UNIT_WIDTH 32
 #endif
 
-#include <thirdparty/pcre2/src/pcre2.h>
+#include <pcre2.h>
 
 static void *_regex_malloc(PCRE2_SIZE p_size, void *p_user) {
 	return memalloc(p_size);