Cleanup: Fix one-definition-rule violations for various structs

This fixes most "One Definition Rule" violations inside blender proper
resulting from duplicate structures of the same name. The fixes were
made similar to that of !135491. See also #120444 for how this has come
up in the past.

These were found by using the following compile options:
-flto=4 -Werror=odr -Werror=lto-type-mismatch -Werror=strict-aliasing

Note: There are still various ODR issues remaining that require
more / different fixes than what was done here.

Pull Request: https://projects.blender.org/blender/blender/pulls/136371

intern/guardedalloc/intern/mallocn_guarded_impl.cc

@@ -107,6 +107,8 @@ typedef struct localListBase {
   void *first, *last;
 } localListBase;
 
+namespace {
+
 /* NOTE(@hos): keep this struct aligned (e.g., IRIX/GCC). */
 typedef struct MemHead {
   int tag1;
@@ -137,6 +139,8 @@ static_assert(MEM_MIN_CPP_ALIGNMENT <= sizeof(MemHead), "Bad size of MemHead");
 
 typedef MemHead MemHeadAligned;
 
+}  // namespace
+
 /* #MemHead::flag. */
 enum MemHeadFlag {
   /**

intern/guardedalloc/intern/mallocn_lockfree_impl.cc

@@ -31,6 +31,8 @@
 
 using namespace mem_guarded::internal;
 
+namespace {
+
 typedef struct MemHead {
   /* Length of allocated memory block. */
   size_t len;
@@ -45,6 +47,8 @@ typedef struct MemHeadAligned {
 static_assert(MEM_MIN_CPP_ALIGNMENT <= alignof(MemHeadAligned), "Bad alignment of MemHeadAligned");
 static_assert(MEM_MIN_CPP_ALIGNMENT <= sizeof(MemHeadAligned), "Bad size of MemHeadAligned");
 
+}  // namespace
+
 static bool malloc_debug_memset = false;
 
 static void (*error_callback)(const char *) = nullptr;