prometheus: fix compilation with EntityBase

[mischief]

What does this implement/fix?

prometheus code seems to lack the header for EntityBase, so fix that.

fixes:

In file included from src/esphome/components/prometheus/prometheus_handler.cpp:3:
src/esphome/components/prometheus/prometheus_handler.h:31:21: error: 'EntityBase' has not been declared
   31 |   void add_label_id(EntityBase *obj, const std::string &value) { relabel_map_id_.insert({obj, value}); }
      |                     ^~~~~~~~~~

Types of changes

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Other

Related issue or feature (if applicable): fixes

Pull request in esphome-docs with documentation (if applicable): esphome/esphome-docs#

Test Environment

• ESP32
• ESP32 IDF
• ESP8266
• RP2040

Example entry for config.yaml:

esphome:
  name: d1-mini
  name_add_mac_suffix: true

esp8266:
  board: d1_mini

logger:

api:

ota:

wifi:
  ssid: !secret wifi_ssid
  password: !secret wifi_password
  fast_connect: true

web_server:
  
prometheus:

Checklist:

• The code change is tested and works locally.
• Tests have been added to verify that the new code works (under tests/ folder).

If user exposed functionality or configuration variables are added/changed:

• Documentation added/updated in esphome-docs.

[mischief]

in https://github.com/esphome/esphome/actions/runs/5094151566/jobs/9157652024?pr=4895:

INFO Installing earlephilhower/tool-pioasm-rp2040-earlephilhower
Tool Manager: Installing earlephilhower/tool-pioasm-rp2040-earlephilhower
Error: Could not find the package with 'earlephilhower/tool-pioasm-rp2040-earlephilhower' requirements for your system 'linux_x86_64'
ERROR PIO assembly failed
Error: Process completed with exit code 1.

i do not think this CI failure is caused by this PR.

[jesserockz]

Thanks @mischief