core: Make converting constructors explicit where applicable

Avoids unwanted implicit conversions. Thankfully, given the large amount
of cleanup in past PRs, only this tiny amount is left over to cover.

src/core/loader/elf.cpp

@@ -190,7 +190,7 @@ private:
     u32 entryPoint;
 
 public:
-    ElfReader(void* ptr);
+    explicit ElfReader(void* ptr);
 
     u32 Read32(int off) const {
         return base32[off >> 2];