[core] Avoid public dtoa.hpp include

include/mbgl/util/string.hpp

@@ -4,8 +4,6 @@
 #include <cassert>
 #include <exception>
 
-#include <mbgl/util/dtoa.hpp>
-
 namespace mbgl {
 namespace util {
 
@@ -22,17 +20,9 @@ inline std::string toString(uint8_t num) {
     return std::to_string(unsigned(num));
 }
 
-inline std::string toString(float num) {
-    return dtoa(num);
-}
-
-inline std::string toString(double num) {
-    return dtoa(num);
-}
-
-inline std::string toString(long double num) {
-    return dtoa(num);
-}
+std::string toString(float);
+std::string toString(double);
+std::string toString(long double);
 
 inline std::string toString(std::exception_ptr error) {
     assert(error);